eGroupWare is many things. It is a very flexible pluggable framework capable of hosting applications like a group calendar, AddressBook, Email, accounting ledger, inventory, and whatever else your can think of. These applications can share a common user store, and a flexible Access Control List (ACL) system to control access and information sharing. At it's core there is the phpgwapi, an API written for php, that provides some very useful functions and tools for developers, including the user store, which has several back ends, database abstraction, and a flexible templating system. Applications are then written using the phpgwapi and phpgw framework to provide web based tools to users.