Earthquakes are measured with the Richter Magnitude Scale developed by Charles F. Richter of California Institute of Technology in 1935. Here is a summary of the Richter scale:
Magnitude less than 3.5: Generally not felt, but recorded.
Magnitude 3.5-5.4: Often felt, but rarely causes damage.
Magnitude under 6.0: At most slight damage to well-designed buildings over small regions.
Magnitude 6.1-6.9: Can be destructive in areas up to 100 kilometers across where people live.
Magnitude 7.0-7.9: Major earthquake. Can cause serious damage over larger areas.
Magnitude 8 or greater: Great earthquake. Can cause serious damage in areas several hundred kilometers across.
Earthquakes are also measured by the Mercalli Intensity Scale which is a longer, more detailed scale. Both the Richter Scale and the Mercalli Scale can be seen on the U.S. Geological Survey website.
Source: United States Geological Survey.