Use this tag for questions related to the Apache http server (any version, but make sure to provide Apache version numbers in the question unless not relevant). Generic webserver related questions should use the tag: webserver. Few of the other Apache related projects have their own tag, don't use this tag for those projects, if they are not http server related.
Apache is an HTTP server that has a large installation base within the UNIX & Linux community.
The server has already gone through several iterations, resulting in similar but not 100% compatible installations and configurations. Make sure to include the particular version number of the Apache version used, in the question.
Apache 2.4 ships with modern distros (state: 2014-10), but older versions (2.2, 2.0 are often included as well for compatibility reasons). Extensions (e.g. 'mod_wsgi`) are available as separately installable packages.