An Expungement is a tool used to clear your record from either a conviction or dismissal. In so doing, after an expungement, a background check will not reveal either your dismissed charge(s) or a conviction. Law enforcement agencies will also destroy the records. Speaking frankly, it never happened.
It is important to note, the court adjudicating you not-guilty or dismissing a criminal charge still means you have a criminal record. Despite a court adjudicating you not-guilty, there will still be a record of the charge which can be found by any background check. What does that mean? An employer, housing authority, or anyone with a few dollars and a computer will be able to see you were once charged with a crime. In order to totally wipe-out a criminal charge, you must expunge the charge.