Answer:
A void contract means that none of the parties to it can legally enforce it. A voidable contract means that one or more but not all of the parties can legally avoid having to perform it. Examples of each (in most jurisdictions - check with yours for exceptions):
1. "adoption contracts": the sale and purchase of babies through private parties without the involvement of the applicable child welfare agencies. This would be void. Nobody has a right to go to court and enforce it.
2. "sale to minor": the sale of a discretionary item to a minor (usually age 18). Say the minor entered into an installment contract to purchase an expensive electric guitar, and even made some of the payments. The minor would have a right to revoke the contract as long as the revocation occurred promptly after the minor attained the legal age of "majority" (ie usually 18), and thus avoid further payments as long as the guitar is returned.