The answer hasn't been fully explained, and probably won't be until the finale.
There seems to be only one crack, which can appear anywhere in time and space. When opened, it releases "pure time energy", which can remove individuals and events from history. At this point, it can only be closed by "feeding" it a complicated space-time event (like a Time Lord, or an army of Weeping Angels).
For some reason that hasn't yet been revealed, it's been following Amy and the Doctor in the TARDIS.
It seems to be connected to Amy's wedding, which will take place on 26 June 2010.