Commonly you may find the letter "P" at the beginning of the size information. For example, the letter "P" denotes that the tire is for a "Passenger Vehicle" meeting the standards for use in the United States, intended for Passenger vehicles.
This is the the tire width measured in millimeters from sidewall to sidewall. In this example, the tire is 225 millimeters wide from sidewall to sidewall.
This is the ratio of the height of the tire's cross-section to its width. The two-digit number after the slash mark in a tire size is the aspect ratio. For example, this P225/60R15 tire, the 60 denotes the height of the sidewall is 60% of the width of the tire.
The letter "R", common in most tires denotes that this tire's construction is "RADIAL" - a popular, reliable and very common construction type.
Your tire's diameter is the size of the wheel it's intended to fit on, measured from one edge to the other. A size P225/60R15 tire is designed for a wheel with a 15" diameter.