When one earthquake happens, we usually see another at a nearby location. Sometimes the change in stress in the mainshock is great enough to trigger aftershocks on nearby faults. As a rule of thumb, we say that aftershocks are other earthquakes triggered at a distance from the mainshock fault no greater than the length of that fault. The aftershock zone of a magnitude 5 mainshock will be under 5 miles across, that of a magnitude 6.5 will be about 20 miles across, while that of magnitude 8 mainshock might be over 200 miles long.
Bigger earthquakes have more and larger aftershocks.
As the magnitude of the mainshock increases, the magnitude of the largest aftershock, on average, increases as well. The relative number of small to large aftershocks does not appear to change with time. In general, an earthquake large enough to cause damage will produce several felt aftershocks within the first hour. The rate of aftershocks dies off quickly with time so even the second day will have many less aftershocks than the first. We call an earthquake an aftershock as long as the rate at which earthquakes are occurring in that region is greater than the rate we saw before the mainshock.