L2j is a project created to develop an alternative Lineage 2 Game Server written from scratch in pure Java to be Cross-platform (multi-platform), L2j Server is the main engine that runs the Game Server and L2j Datapack is the actual content of your server, having both Engine and Data separate from each other allows you to modify the Datapack to your needs and with it you could even create a new Original MMO.
L2j gives you the possibility to legally host a Game Server compatible with this popular Korean MMO created by NCSoft.
L2j is still unfinished and some features are still not implemented, but the lastest version of the Server is one of the most robust, stable and powerful ever made, the L2J Dev Team is working hard on implement the missing features and add new content.
If you want to learn more about L2j you can start with browing the Community Portal
L2J Server is distributed under the terms of GNU GENERAL PUBLIC LICENSE VERSION 3 in a hope that open source model is the best for developing quality software giving everyone a possibility to participate on development by submitting the code.