This document discusses open source software. It begins with an introduction on how open source software can be used to provide library services through websites. It then defines open source software as software with source code that is freely available under an open source license for modification and redistribution. The document categorizes some common types of open source library software and analyzes the advantages and disadvantages of open source software. It concludes that the evolving nature of open source software gives it adaptive flexibility which is important for its future.