In a federal system of government, there are different sources of power, which tends to be more widely dispersed. In the United States, for example, which has a federal system, political power emanates from both the center—the federal government in Washington, DC—and the individual states. Both federal and state authorities have the power to make laws, though their respective roles are different, with each center of power having specific competencies.
In a unitary system of government, on the other hand, there is just one source of power at the center. Though power and autonomy can be devolved to the local level, such power is entirely dependent on the central government and as such can always be revoked. Unlike a federal system, devolved power in unitary states doesn't enjoy any degree of constitutional protection. In the United Kingdom, for example, the devolved Scottish Parliament and Welsh Assembly have considerable responsibility over their own affairs. Yet ultimate political sovereignty still resides with the UK Parliament at Westminster. This means that Parliament could abolish these devolved bodies if it wanted to, as it could with all forms of local government. Such an exercise of political authority by the central government would not be possible in a federal system.