An expungement is an erasure of a criminal offense from your "record". In Illinois this process is created statutorily. Once a criminal offense becomes a conviction there is no means to erase it from your record. This is true for both misdemeanor and felony convictions. However, this does not mean you cannot negotiate with the prosecution to change the disposition after the fact.
An expungement is effective when a person has satisfied a term of supervision (or first offender probation), or has simply had the charge dismissed. After successfully completing the term of supervision the person needs to wait the statutorily proscribed period (usually 2 years) before filing the petition to expunge. The same holds true with completion of first offender probation (except the wait is 3 years).
The expungement petition must be filed with the circuit clerk in the county where the case was filed. This process takes approximately 4 months to fully complete. By law, the State's Attorney, Illinois State Police, and the local police have 30 days to file an objection to this process after they are served with the petition. If there is no objection and the person is entitled to expungement the proposed order is entered by the judge and copies of that order are sent to the Illinois State Police and local police departments.
The circuit clerk then seals the file and removes it from public view at the county level. Not all charges can be expunged. It is important to consult a knowledgeable attorney prior to investing money in this endeavor. Commonly I am asked whether a DUI can be expunged. The answer is NO. This is one example of an offense that cannot be expunged.