Common features
The rack of web servers hosting the My Opera Community site on the Internet. The Opera Community rack, as seen to the left. From the top, user file storage (content of files.myopera.com), "bigma" (the master MySQL database server), and two IBM blade centers containing multi-purpose machines (Apache front ends, Apache back ends, slave MySQL database servers, load balancers, file servers, cache servers and sync masters.Although web server programs differ in detail, they all share some basic common features.

HTTP: every web server program operates by accepting HTTP requests from the client, and providing an HTTP response to the client. The HTTP response usually consists of an HTML document, but can also be a raw file, an image, or some other type of document (defined by MIME-types); if some error is found in client request or while trying to serve the request, a web server has to send an error response which may include some custom HTML or text messages to better explain the problem to end users.
Logging: usually web servers have also the capability of logging some detailed information, about client requests and server responses, to log files; this allows the webmaster to collect statistics by running log analyzers on log files.
In practice many web servers implement the following features also:

Authentication, optional authorization request (request of user name and password) before allowing access to some or all kind of resources.
Handling of static content (file content recorded in server's filesystem(s)) and dynamic content by supporting one or more related interfaces (SSI, CGI, SCGI, FastCGI, JSP, PHP, ASP, ASP .NET, Server API such as NSAPI, ISAPI, etc.).
HTTPS support (by SSL or TLS) to allow secure (encrypted) connections to the server on the standard port 443 instead of usual port 80.
Content compression (i.e. by gzip encoding) to reduce the size of the responses (to lower bandwidth usage, etc.).
Virtual hosting to serve many web sites using one IP address.
Large file support to be able to serve files whose size is greater than 2 GB on 32 bit OS.
Bandwidth throttling to limit the speed of responses in order to not saturate the network and to be able to serve more clients.

Origin of returned content
The origin of the content sent by server is called:

static if it comes from an existing file lying on a filesystem;
dynamic if it is dynamically generated by some other program or script or Application Programming Interface called by the web server.
Serving static content is usually much faster (from 2 to 100 times) than serving dynamic content, especially if the latter involves data pulled from a database.

Path translation
Web servers are able to map the path component of a Uniform Resource Locator (URL) into:

a local file system resource (for static requests);
an internal or external program name (for dynamic requests).
For a static request the URL path specified by the client is relative to the Web server's root directory.

Consider the following URL as it would be requested by a client:

http://www.example.com/path/file.html
The client's web browser will translate it into a connection to www.example.com with the following HTTP 1.1 request:

GET /path/file.html HTTP/1.1
Host: www.example.com
The web server on www.example.com will append the given path to the path of its root directory. On Unix machines, this is commonly /var/www/htdocs. The result is the local file system resource:

/var/www/htdocs/path/file.html
The web server will then read the file, if it exists, and send a response to the client's web browser. The response will describe the content of the file and contain the file itself.

Although web servers programs differ in detail, they all do share some basic common features.

HTTP: every web server program operates by accepting HTTP requests from the client, and providing an HTTP response to the client. The HTTP response usually consists of an HTML document, but can also be a raw file, an image, or some other type of document (defined by MIME-types); if some error is found in client request or while trying to serve the request, a web server has to send an error response which may include some custom HTML or text messages to better explain the problem to end users.
Logging: usually web servers have also the capability of logging some detailed information, about client requests and server responses, to log files; this allows the webmaster to collect statistics by running log analyzers on log files.
In practice many web servers implement the following features also:

Authentication, optional authorization request (request of user name and password) before allowing access to some or all kind of resources.
Handling of static content (file content recorded in server's filesystem(s)) and dynamic content by supporting one or more related interfaces (SSI, CGI, SCGI, FastCGI, JSP, PHP, ASP, ASP .NET, Server API such as NSAPI, ISAPI, etc.).
HTTPS support (by SSL or TLS) to allow secure (encrypted) connections to the server on the standard port 443 instead of usual port 80.
Content compression (i.e. by gzip encoding) to reduce the size of the responses (to lower bandwidth usage, etc.).
Virtual hosting to serve many web sites using one IP address.
Large file support to be able to serve files whose size is greater than 2 GB on 32 bit OS.
Bandwidth throttling to limit the speed of responses in order to not saturate the network and to be able to serve more clients.

Load limits
A web server (program) has defined load limits, because it can handle only a limited number of concurrent client connections (usually between 2 and 60,000, by default between 500 and 1,000) per IP address (and IP port) and it can serve only a certain maximum number of requests per second depending on:

its own settings;
the HTTP request type;
content origin (static or dynamic);
the fact that the served content is or is not cached;
the hardware and software limits of the OS where it is working.
When a web server is near to or over its limits, it becomes overloaded and thus unresponsive.

Overload causes

A daily graph of a web server's load, indicating a spike in the load early in the day.At any time web servers can be overloaded because of:

Too much legitimate web traffic (i.e. thousands or even millions of clients hitting the web site in a short interval of time. e.g. Slashdot effect);
DDoS (Distributed Denial of Service) attacks;
Computer worms that sometimes cause abnormal traffic because of millions of infected computers (not coordinated among them);
XSS viruses can cause high traffic because of millions of infected browsers and/or web servers;
Internet web robots traffic not filtered / limited on large web sites with very few resources (bandwidth, etc.);
Internet (network) slowdowns, so that client requests are served more slowly and the number of connections increases so much that server limits are reached;
Web servers (computers) partial unavailability, this can happen because of required / urgent maintenance or upgrade, HW or SW failures, back-end (i.e. DB) failures, etc.; in these cases the remaining web servers get too much traffic and become overloaded.

Overload symptoms
The symptoms of an overloaded web server are:

requests are served with (possibly long) delays (from 1 second to a few hundred seconds);
500, 502, 503, 504 HTTP errors are returned to clients (sometimes also unrelated 404 error or even 408 error may be returned);
TCP connections are refused or reset (interrupted) before any content is sent to clients;
in very rare cases, only partial contents are sent (but this behavior may well be considered a bug, even if it usually depends on unavailable system resources).

Anti-overload techniques
To partially overcome above load limits and to prevent overload, most popular web sites use common techniques like:

managing network traffic, by using:
Firewalls to block unwanted traffic coming from bad IP sources or having bad patterns;
HTTP traffic managers to drop, redirect or rewrite requests having bad HTTP patterns;
Bandwidth management and traffic shaping, in order to smooth down peaks in network usage;
deploying web cache techniques;
using different domain names to serve different (static and dynamic) content by separate Web servers, i.e.:
http://images.example.com
http://www.example.com
using different domain names and/or computers to separate big files from small and medium sized files; the idea is to be able to fully cache small and medium sized files and to efficiently serve big or huge (over 10 - 1000 MB) files by using different settings;
using many Web servers (programs) per computer, each one bound to its own network card and IP address;
using many Web servers (computers) that are grouped together so that they act or are seen as one big Web server, see also: Load balancer;
adding more hardware resources (i.e. RAM, disks) to each computer;
tuning OS parameters for hardware capabilities and usage;
using more efficient computer programs for web servers, etc.;
using other workarounds, especially if dynamic content is involved.

Historical notes

The world's first web server.In 1989 Tim Berners-Lee proposed to his employer CERN (European Organization for Nuclear Research) a new project, which had the goal of easing the exchange of information between scientists by using a hypertext system. As a result of the implementation of this project, in 1990 Berners-Lee wrote two programs:

a browser called WorldWideWeb;
the world's first web server, which ran on NeXTSTEP.
Between 1991 and 1994 the simplicity and effectiveness of early technologies used to surf and exchange data through the World Wide Web helped to port them to many different operating systems and spread their use among lots of different social groups of people, first in scientific organizations, then in universities and finally in industry.

In 1994 Tim Berners-Lee decided to constitute the World Wide Web Consortium to regulate the further development of the many technologies involved (HTTP, HTML, etc.) through a standardization process.

The following years are recent history which has seen an exponential growth of the number of web sites and servers