Apache is an open source web server software that has the largest market share. It was started in 1995 by a group of developers as an open source alternative to proprietary web servers. Apache has many advantages, including being cross-platform, customizable through user-created modules, and providing strong security features like SSL authentication. It also supports languages like PHP, MySQL, and Perl. While it is very robust and customizable, one disadvantage is challenges with name-based virtual hosting over SSL. Overall, Apache continues to be successfully developed and improved through an open community.