Expunging or setting aside a conviction, in essence, is clearing the conviction, and record of arrest, completely off your record. However, there are hurdles and statutes that must be satisfied in order to be eligible.
Firstly, the crime must be one which is eligible for expungement. No traffic offenses, such as DUIs or reckless driving, are eligible for expungement. Most A and B Felony convictions likewise are not expungable. Beyond that, you must wait 3 years from the date of conviction before you may be eligible for expungement.
Once you have determined that it has been at least 3 years since the date of conviction, you must look back 10 years to be sure you have no other convictions. The law requires you be conviction free for 10 years before you can expunge a conviction.