What is an annulment and what is its difference from divorce?

Annulment is an unusual remedy. It voids a void or voidable marriage for very unusual circumstances none of which appears to be present here.

A nullity (annulment) is available under very limited circumstances when the marriage is either void or voidable for a statutory reason such as bigamy or fraud. Your situation does not appear to qualify for a Nullity and a regular divorce action should be filed.

A divorce divides the community property and obtains a Judgment to terminate the marriage. An Annulment goes back and determines there never was a marriage. Therefore, there is nothing to divide, including children custodial time. If there are children, then a Paternity action is required to deal with the children. An Annulment is usually difficult to obtain requiring the proof of fraud in the obtaining of the marriage, illegal marriage (one person was and is married at the time of the marriage at issue (bigamous), unsound mind, force, physical incapacity. If cohabitation continues after finding out one of the reasons allowed for an Annulment, then that defense may become waived.

An annulment undoes the marriage so that it never happened. It may be a better option, although they are hard to obtain (you have to prove fraud in this case). Unless you got something of significant value in the time you were together, it would be easier to just get a divorce. You should definitely talk with an attorney before filing for anything.