phpBB3 is the latest addition to the phpBB community. While the old phpBB2 was a simple but functional forum system that every body knows, phpBB3 is a lot more powerful and comes as competition to the costly InvisionPowerBoard (also known as IPB). phpBB3 is opensource, distributed under GNU/GPL license. All you need to run phpBB3 is a web server (I recommend Apache), PHP engine in it and a database server of course (I recommend MySQL). Have a nice forumming experience with phpBB3!