Each tire has identifiers written on them: cross section width, aspect ratio, and wheel diameter.
The aspect ratio is dependent on your tire width. In the picture to the left the aspect ratio is 50% of the 225 millimeter width. From rim to tread it's 112.50 millimeters.
Different sizes can be put on your vehicle in place of your current size, so long as they fit the rim and don't rub on the wheel well. For example, 215/50/16 would be an alternative to 225/50/16.