What is Apache Server?
Apache 2.0 is a web server that handles page requests from a browser (client) and sends these requests back to the user in information that is useful (usually HTML code) to them. Apache has become widely succesful and is used by the vast majority of Linux hosted servers.
Many popular modules are also available for Apache such as the mod_rewrite module which allows users to rewrite URLs, redirect users and much more. Apache is widely supported and very effecient, leading to its prominent use in the webhosting/server world. The current most stable version of Apache is Apache 2.0.