2 Answers

Contract is a legally enforceable agreement between two or more parties with mutual obligations, which may or may not have elements in writing.  In some states, the statute of fraud requires that most agreements be in writing and acknowledged by the parties involved.  For example, you can rent my house for $1,000 per month for one year if you pay all the utilities and expenses to maintain the house except real property tax.

A unilateral contract is a one-sided contract.  It only requires the person giving the offer to perform.  For example, I will give you an option (contract) to buy my house within 1 year if you can close the deal before the end of the 12th month at the price of $60,000.  You don't have to buy my house but if you exercise the option to buy, I must sell to you.  Only one party is required here to perform.

a legal agreement made between two or more parties.