In many respects, the terms complaint and petition are interchangeable. Both are pleadings -- they initiate legal proceedings. One party files the document with the court and the other party responds with a written answer. There’s a subtle difference between them, but not all states recognize it, particularly in the area of family law.
Similarities in Pleadings
Both complaints and petitions identify the person or entity filing the lawsuit and the person or entity he’s suing. With a petition, the filing party is the petitioner; with a complaint, he’s the plaintiff. The person being sued is either the respondent in a petition or the defendant in a complaint. Both documents explain the claim being made, such as a request for divorce or monetary judgment. Both explain why the petitioner or plaintiff feels that he’s entitled to these things.
Differences in Pleadings
Technically, a complaint asks the court to award monetary damages or obligate the defendant to take some action. A petition only asks the court to make a ruling on something, such as the termination of a marriage. But terminating a marriage usually involves monetary issues as well, so when it comes to divorce, some states use petitions and some use complaints. In some states, complaints are filed in contested matters and petitions are used when a divorce is uncontested. If your lawsuit involves damages -- financial reimbursement for some civil wrong -- you would almost certainly have to file a complaint.