Here's how it breaks down for a 33/12.50/15.
The first number is the tire diameter, so 33's would be 33" tall.
The next number is tread width, so the tread width is 12.5".
Finally the 15 this is wheel (you may call it rim) diameter.
Most tire manufacturers recommend going no more than 4" wider on the tread than the width of the wheel. For example if you want to run the 33/12.50/15's the minimum wheel width would be about 8", for the 33/12.50/15's a 10" wide wheel works pretty well.
Of course then you have backspacing (think depth of the hub inside the rim of the wheel, this affets how far the wheel tire combo are tucked into the axle), and bolt patterns. But since you didn't ask about them, and I am really tired I won't even try to explain them today :?
Hope this helps, and by the way on metric tires the only number that corresponds is the last number. I don't recall the what the rest of the numbers represent on metric.