The Richter Scale is based on a base-10 logarithmic scale, ranging from 0 to 9+. On the scale, each increase in number represents an earthquake 10 times more powerful, a Richter 3 quake is 10 times larger than a Richter 4 and a Richter 5 is 10 times greater than a Richter 4; the result is that a Richter 5 is 100 times greater than a Richter 3.
The largest earthquake currently on record is the Great Chilean Earthquake of May 22, 1960, with a magnitude of Richter 9.5.
Here's a great graph from the USGS:
If you feel a quake, report it online at USGS "Did you feel it?"!