Using the example above - 285/75/16 - here you go:
285 = tread width, expressed in millimeters.
75 = aspect ratio. This can be confusing. It's the ratio, expressed as a percentage, of the sidewall height to the tread width. In this example, the sidewall height is 75% of 285, or 213.75mm.
16 = wheel diameter.
I'll take a wild guess that your size is something like 285/50/20. The size will be displayed on the sidewall. The reason I say that aspect ratio can be confusing is that a lot of people mistake it for width. For example, a common mistake is that a 50 series tire is wider than a 60 series tire. Not necessarily true, and you have to use the tread width to say for sure.
Lots of other boring stuff on tires - I won't put you to sleep any longer. I recommend checking out the tutorial at
http://www.tires.com if you're interested.
Take care,
Jerry