Earthquake swarms are events where a local area experiences sequences of many earthquakes striking in a relatively short period of time. The length of time used to define the swarm itself varies, but the United States Geological Survey points out that an event may be on the order of days, weeks, or months. They are differentiated from earthquakes succeeded by a series of aftershocks by the observation that no single earthquake in the sequence is obviously the main shock.
One example is along the Cerro Prieto fault near Mexicali, BC in Mexico where over 500 quakes and aftershocks hit in February, 2008. Another is a swarm that's been dubbed "The Mogul earthquake sequence" that began in February 2008 near Reno, Nevada and is still occurring as of August 2008. Between February and April the swarm produced more than 1,000 quakes of small magnitude, although the largest measured 4.7.