At a high level, the difference is that crimes against humanity involve a widespread and concerted attack against the civilian population, but it might not be based on, let's say, their identity as a group.
In the case of genocide, there's the fact that you are trying to destroy a group in whole or in part and that there's an intent to do so.
They are thus legally different, and the intent element of genocide in particular can be a bit challenging to establish.