Let's take a P225/45R17
The P refers to Passenger tire. Sometimes you'll see LT on a 3/4 ton truck for "Light Truck"
The first number (225) is the section width, in mm. The number after the slash (45) is the tire side wall height ratio, in percentage of the section width. The R stands for Radial construction, and the next number (17) is the wheel diameter.
http://www.tirerack.com/tires/tirete....jsp?techid=46