Apache is the colloquial name for the Apache HTTP Server project, run by the Apache Foundation. Apache is a web server that processes code and presents the human viewable website to the end user through the HTTP protocol. When you type in https://actionspec.com – Apache is what processes the php and markup for final presentation to you, the end user.
Apache runs on mostly Linux servers, but also runs on Mac OS X, and Windows as well.
Apache is the “A” in LAMP Stack.