Contracts can be classified into various types based on their nature and enforceability. Common classifications include:

1. **Based on Formation:**

   - **Express Contract:** Terms are explicitly stated, either orally or in writing.

   - **Implied Contract:** Terms are inferred from the parties' conduct or the circumstances.

2. **Based on Enforceability:**

   - **Valid Contract:** Meets all legal requirements.

   - **Void Contract:** Lacks legal effect from the beginning.

   - **Voidable Contract:** One party has the option to enforce or void the contract.

   - **Unenforceable Contract:** Valid but cannot be enforced due to legal reasons.

3. **Based on Performance:**

   - **Executed Contract:** Fully performed by both parties.

   - **Executory Contract:** Not fully performed by one or both parties.

4. **Based on Number of Parties:**

   - **Bilateral Contract:** Promise for a promise (two parties involved).

   - **Unilateral Contract:** Promise for an act (one party makes a promise, and the other performs an act).

5. **Based on Time:**

   - **Periodic Contract:** Repeats at regular intervals.

   - **Fixed-term Contract:** Lasts for a specific period.

6. **Based on Nature of Obligation:**

   - **Unconditional Contract:** No conditions attached.

   - **Conditional Contract:** Depends on the occurrence of a specific event.

Understanding these classifications helps in interpreting and applying contract law appropriately.