Welcome to Fluther.
The depth and breadth of a “background check” varies depending on 1) how much trust will be placed in you: a CIA operative normally gets a deeper and more thorough background check than a supermarket cashier, and 2) by the willingness of the person doing the hiring (and requesting the check) to pay for it. Again, the federal government is more willing to spend the money to marshal the resources and pay for a background check of a person’s entire life, to the extent that it can be investigated: questioning former neighbors and working colleagues, ranging over an adult’s complete history to check that it’s in accordance with what he has claimed as a history, looking for discrepancies, etc.
In your case, the investigator will probably only do a records and previous employment check on you. They will look to see if you have any criminal complaints or convictions, and talk to prior employers (if any) to see if there were problems with trust associated with that.
Every background check that I’ve ever known about (to my knowledge I haven’t had any run on me, but we’ve had employees whose “casual” employment in the nuclear field required an investigation in the normal course of employment) has been specified: “A five-year background check”, for example. If they find something worth noting, such as the fact that you said you were doing one thing five years ago but you were doing something else entirely, then they may dig further to see if this can be explained as “oversight / omission” or “deliberate falsification”.