Contracts can be classified into several types based on their formation and enforceability. The two main categories are express contracts, where terms are clearly stated, and implied contracts, which are formed through actions or circumstances. Additionally, contracts can be classified as bilateral, involving mutual promises, or unilateral, where one party makes a promise in exchange for an act. Another way to categorize contracts is by their enforceability. Valid contracts are legally binding, while void contracts have no legal effect. Voidable contracts can be enforced at one party's discretion, and unenforceable contracts cannot be enforced due to legal reasons, such as lacking a required form.