There are several types of contracts:
1. A valid contract is enforceable by law when all essential elements are present.
2. A voidable contract can be voided when consent is not free, such as under threat.
3. A void contract has no legal effect.
Contracts can also be express (verbal or written), implied (based on actions), quasi (not by agreement but recognized by law), executed (both parties fulfilled obligations), or executory (obligations still need to be performed).