This wiki contains some basic information about MediaPortal 2.
It still lacks a lot of details, but is constantly improved. If you are interested in contributing to the documentation (or in any other way), please take a look at the contribution guide.
Besides the Wiki additional information can also be found in
Feel free to open a new thread in the forum if you can' find the information you are looking for after using the search functionality in Wiki, forum and Issue Tracker.