The parties are both under an obligation, but their obligation is not mutual, and in other words one obligation is not exchanged for the
other.

Only one of the parties is under the burden of fulfilling a consideration while the other party does not owe any consideration.

There are mutual promises between the two parties and they involve an exchange of equivalent mutual obligations.

Contract of agency and loan contracts can be given as examples.

Sales contract, rental contract and employment contract can be given as examples.