Common features

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 it, 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.
http://en.wikipedia.org/wiki/Web_server