OpenJazz is a Sonic-type game. It is a port for Pocket PC of Open Jazz, a "free, open-source version of the classic Jazz Jackrabbit™ game" from Alister Thomson.
Just like any Sonic-type game, you have to run, jump, destroy enemies and collect points. Music is great and not so repetitive as it uses to be in this kind of game.
Please note that the version of OpenJazz this port is based on is still pre-alpha and has some bugs. So, before reporting a bug please check the original OpenJazz FAQ here and if this bug also appears in the desktop version of OpenJazz available here.