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). Non-apache web server-related questions should use the tag: webserver.
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. Include the version of Apache in your question unless you are sure that it is irrelevant.
Apache ships with modern distributions, and extensions (e.g. 'mod_wsgi`) are available as separately installable packages.