In a nutshell, GooSync facilitates an over the air synchronization of your Google calendars and contacts with up to 4 mobile devices. It is possible for each device to have separate settings to control specifically which calendars or contact categories are synchronized, but they all must connect to the same Google account.
A free account is still available but multiple device support is just one of the really nice extra features of a Premium account. It runs about $40/year, a real bargain in my opinion for the ability to stay up to date no matter where I am.
Is multi-device support important to you?