The more familiar number is the magnitude of the earthquake. This number represents how much energy was released. It is based on a logarithmic scale, so a magnitude 7.1 earthquake releases 10 times as much energy as a 6.1 earthquake. Earthquakes of magnitudes less than 3.5 are generally not felt, but they are recorded. A major earthquake is considered to be magnitude at least 7.0, while a great earthquake is magnitude 8.0 or greater.
Intensity is a measure of how forceful the earthquake feels in a certain area.
The intensity will vary based on how far away from the epicenter you are, as well as the nature of the ground in the area. For example, an earthquake will feel more intense on loose fill than on solid granite. Here are some other information sources and related links.