Every crack is different. Some contain "the silence," some exude time energy that erases things from existence, and others are passageways to other worlds.
Some, the most dangerous, are a void or hole in time. Causing them to swallow up events and people causing them to cease to exist ever at all. As shown, throw enough into them of a major time "event" or "being" and they are appeased and stop for a while. This doesn't repair the crack however and we have yet to see more details on just where this crack is going to lead the Doctor.
Overall, it's usually stuff from other universes, since the 2nd Big Bang caused them, and they're the weakest points of our universe that stuff tries to get through. Like Gallifrey, which is trapped in a pocket universe and tried to get back to our universe in "Last Christmas".