Tire size question - OK, so call me stupid ;-)

[Tom Winter]

This is so basic that I'm almost embarrassed to ask, but I'm going to do so
anyhow.

My wife's BMW needs new tires, and the size of her current (almost bald)
rubber is 205/50/17.

Ok, so what exactly do these numbers mean? How do I know how to sub a
different sized tire on the rim, and if it will fit?

Which measurements are important, which can be different? (within reason).

Because she will be selling the car soon, I want to put some decent used
rubber on the car for the 1-2 months she'll be driving it. As an aside,
hopefully I can convince her to ditch the BMW brand, but although she likes
my Audi, she's pretty stuck on BMW

MAC: Yeah, I know. I should know this stuff, but I've always run stock rims
and the appropriate sized tire on my Audis, so I've never bothered to learn
anything. At least, unlike my wife, I can change my own oil and fix a flat!
LOL! 

Thanks in advance for cluing me in to this esoteric knowledge.

Tom

'95 S6 Avant