The San Jacinto fault is a major structure in Southern California, an active right lateral strike slip fault that trends parallel to the San Andreas. It has produced damaging quakes in the past, most notably in 1899 and 1918 which caused serious damage in Hemet and San Jacinto. At least 10 historic quakes have exceeded magnitude 6.
The quakes today are not nearly as large (a magnitude 5 quake has only 1/30 the energy of a magnitude 6 event), but all earthquake swarms on active fault systems should be noted carefully. Most of the time nothing else happens, but on occasion these swarms may presage a larger event. If you live in the region, it is a good time to evaluate your earthquake preparedness, storing water, flashlights, food, batteries, first aid kit, and radio, and having a family emergency plan.
For more information on earthquake hazards in California refer to this site from the USGS: http://earthquake.usgs.gov/prepare/ . For a look at the historical record of large earthquakes in Southern California, check here: http://www.data.scec.org/significant/index.html . For an interactive map that provides the latest data on earthquakes in California (and other parts of the world), check here: http://earthquake.usgs.gov/earthquakes/map/. If you felt any of the earthquakes and want to report your experience, check out Did You Feel It?.
As is often the case, the preliminary data is a bit uncertain, and the latest official count is one magnitude 4.7 quake, which as of 1:30 PM has been followed by over 200 aftershocks.