An enforceable contract is a normal contract that has legal effect.
A void contract has no legal effect. Example: a contract to commit crime.
A voidable contract has legal effect unless one party (e.g., a minor) chooses to void it.
An unenforceable contract is one that has some legal status but no conventional means for a party to assert his rights. Example: The statute of limitations precludes a party from bringing an action in court after a specified time period has elapsed. While corrective measures cannot make a void contract enforceable, an unenforceable contract can sometimes be validated by corrective action of the parties (e.g., memorializing an agreement in writing to satisfy the Statute of Frauds).