In a unitary government, all powers are concentrated in one central authority while in a federal government powers are distributed between the central and state governments. Both of them enjoy coordinate powers.
A unitary government has a supreme central legislature, while in a federal form of government there are separate legislatures and at the central and state levels. The constitution here is supreme, and lays down the powers of the two sets of government.
In a unitary form of government, the constitution is not the outcome of an agreement and can be changed by its own supreme authority. In a federal government the constitution is the outcome of an agreement, and hence it can only be amended by a special procedure.
In a federation there is an independent judiciary to resolve conflicts between the centre and the states. There is no such mechanism in a unitary government. In a unitary form of government a unitary system of law prevails, but in a federation citizens have to obey two sets of laws.