It was once the case that all spare tires were identical to the tires already on your car. When you got the inevitable flat, you'd simply change the tire and be on your way, getting your tire repaired at your convenience.
Nowadays spare tires have shrunk in size - some cars don't even have one! We'll walk you through the different sized spares your car may have.
Full size spare
If you want to be well prepared, this is the spare for you. Keep a full sized spare in (or on) your car and change it when needed. A flat tire is simply a minor inconvenience, not something that ruins your plans while you immediately get a new tire. Not many automobiles come with a full sized spare anymore (they're more common in trucks and SUVs but are disappearing there as well). If a full size spare appeals to you, purchase an extra rim and tire that matches your other four. You'll sacrifice space and weight, but you'll be well prepared. Put this tire into your regular tire rotation and you'll extend the life of all your tires.
The alternate full size spare is a tire that matches the rest, but is on a plain steel rim. It'll obviously look out of place on your car, but you'll still get some of the benefits of a full sized spare.
Compact spare (or donut)
This is what most cars have these days. These are small tires strictly for getting you on the road long enough until you can get your flat fixed or replaced. They're usually recommended to be driven on for only 50 miles and not over 50 mph.
If you need to buy a new donut, the best way is to contact your dealer. Not all compact spares are the same size and vary greatly! Spares from the dealer are pretty expensive, so if you want to save money one option is the junkyard. Many cars that are wrecked have the spare intact. The junkyard may just have the exact spare for your car! If not, make sure you measure your donut well - especially the bolt pattern so you can find one that fits.
Collapsible Spare
In the name of the ever shrinking spare, some cars now have a collapsible spare. This where the tire itself is deflated and collapsed along the rim. You must inflate it before installing it! These aren't too common, but you should definitely check your tire to see if it's a collapsible spare, so you won't be in for a surprise on the side of the road. If you plan on replacing one of these, you should probably go through the dealer.
No spare
Now for the smallest size of all - none! Some car manufacturers are doing away with spare tires altogether and opting for run-flat tires. These tires resist inflation, and can be driven on briefly, sometimes for over 100 miles! It's best to check your manual to see if you have no spare tire so you're not surprised.
In all cases, its best to consult your car's manual for the recommend spare procedure. Check your spare and your tools, so you're well prepare for that inevitable flat. Good luck!