Which version of GarageBand are you using?
Basically, you can set a track as a groove track, and match (synchronise) the timing of other tracks in the project to it. When you play the project, the timing of matched tracks adjusts to match the timing of the groove track
Only one track in a project can be a groove track. You can select which tracks in the project match the timing of the groove track. When a track is set to match the groove track, time quantization is unavailable for that track.
For GarageBand 10.0 see this Help page: http://help.apple.com/garageband/mac/10.0/#gbnd30cc2692
For GarageBand '11 see this: http://help.apple.com/garageband/index.html#gbndc36de615
I do not use this feature frequently, but for the projects (simple songs with a vocal track and a guitar) I tried it, it works well. Just try it, to see, if it works with the kind of music projects you are doing.