What are the differences between unitary and federal systems?

According to the Norwegian political scientist Xyvind Xsterud, the difference between a unitary and a federal state is not that one is more decentralized than the other, but that the former is decentralized through legislation whereas the latter is decentralized by constitution. In a federation, certain matters are thus constitutionally devolved to local units, and the central government cannot unilaterally revoke this decentralization, as it can in a unitary state.