rebuild-mvc (default branch): the latest branch, the code for the HistoryMap extension. This is the latest version that tries to add 1) saving user session to a remote server, 2) add a better UI, but these are not quite finished. The 'mvc' in the name stands for 'model-view-controller', which is a popular way to structure the code, and something we try to follow in the 'rebuild'.
master: this is the oldest branch, contains the source code of the SenseMap extension. It can only save files locally, and hasn't been updated for a while.
All the other branches are somewhere in between 'master' and 'rebuld-mvc'.