A car lien is a document given legal authority that confirms a claim that the holder of the lien has upon a specifically identified vehicle. They are a by-product of 'contract law' which protect the right of individuals and companies.
1 Additional Answer
A lien on a car is a legal document stating that bank has a right to repossess a car if a lender does not pay as planned. It is usually done for vehicles that are purchased using loan money and serves the purpose of protecting the lender in case of default. In addition, a vehicle with a lien cannot be sold unless the loan amount has been cleared.