The Apache HTTP Server is an Open Source Web server, which originated from a series of patches to the NCSA HTTPd. This is how the name was coined, as it was a patchy HTTPd. Since then, Apache has been entirely re-written and no longer includes the NCSA source code.
Apache is the world's most popular Web server, which is run on 54.90% of all Web servers according to the Netcraft survey conducted July 2010.
Apache is very well documented and most functionality is described in detail within this documentation.