What is a license?
Within the context of copyright law, a license is where a copyright owner transfers something less than all of his exclusive rights in a protected work to another party. This can be a transfer of just some of the rights (for example, the right to distribute but not the right to display) or a transfer of nonexclusive rights (for example, the new party can distribute the work, but the copyright owner can also continue to distribute the work). An assignment, on the other hand, transfers all of the exclusive rights (except for moral rights) to the other party.
Unlike an assignment, a license does not need to be written or signed by the copyright owner, although it should be.