An adversary proceeding in bankruptcy is a separate lawsuit filed within the bankruptcy case. Like most lawsuits, it starts when someone (the creditor, the bankruptcy trustee, or you) files a complaint. Many bankruptcies go through to completion and discharge without any adversary proceedings. But not so in others.
A creditor or the bankruptcy trustee might bring an adversary proceeding to challenge the dischargeability of a particular debt -- alleging that you incurred it through fraud. Or the trustee might seek to regain property that you transferred or sold to someone else prior to your bankruptcy. You can bring an adversary proceeding too. For example, in many districts, you can only get rid of junior liens on real estate through an adversary proceeding.
Read the articles below to get details on how adversary proceedings work, common types of adversary complaints, and more.