Here are some things to look for – or look out for – when you get a report on the vehicle from a site like CarFax. None of these things is necessarily a reason not to buy a car, but you shouldn't make a decision without asking about anything you see on a vehicle's history:
The more garages a car's been in, the less likely it's been lovingly cared for all its life. Not everyone is as diligent about car care as you are. Rental cars and former taxis, for example, will often have undergone a lot of abuse, although they tend to be quite inexpensive.
Out of state
Some parts of the country are more car-friendly than others. Winter storms (with their accompanying salty roads) can be rough on cars, as obviously can floods, excessive heat or even sea air. Cars that have been where these are common may have hidden damage.
Name and description
Be sure the car in the report is the same as the car you're looking at. Carefully reviewing the vehicle description is one way to avoid various types of vehicle fraud, like VIN cloning. A cloned vehicle involves using a vehicle identification number (VIN) from a legally owned, non-stolen vehicle to mask the identity of a similar make/model stolen vehicle. Carfax reports include detailed descriptions of the vehicle, so you can make sure the car you're reading about is the same as the one you're looking at.
Keep an eye out for records of body work that might indicate a prior unreported incident.