By far the most common reason for a car or truck to get labeled "salvage" is involvement in a crash. But in some states, flood and fire damage are also listed as salvage [source: CARFAX]. And these flood and fire salvages are tricky -- things that seem to be working can and do fail, and it's hard to predict what, exactly, needs to be done to bring these flood and fire cars into reasonably reliable condition. The gist: Buyer beware of cars or trucks salvaged due to fire or water damage.
Similarly, there's "good" and "bad" crash damage. First, know your categories for rating vehicle damage -- you want to be lower in the alphabet. A is terrible, B is bad, and categories C and D designate workable damage (not that you're likely to see C or D cars in the salvage lot). The rule of thumb is to stick with body damage, or at the most, damage to the working parts. Stay away from bent frames, which, no matter how good a car looks, can be a one-way ticket to a headache.