There's a big difference between a "license" and a full "assignment" for intellectual property rights.
An intellectual property assignment usually means a complete transfer of intellectual property from one party to another.
In contrast, a license is usually a subset of rights to another party, which can only use the intellectual property in specified ways and for a specified length of time.
A copyright license might say, for example, that a party can only use a certain image or song, perhaps only in the United States or only on a certain webpage.
A full assignment of an image would give a party the right to use the image in whatever way he or she wants, or to create derivatives of the copyright.
Sometimes people speak of "exclusive" licenses, but this phrase should be unbundled. An exclusive license to use certain intellectual property may simply mean that only one party can use it; but it still might be constrained by length of time, geographic location, or other limitations.
A fully exclusive, non-revocable, perpetual license comes close to being a full assignment, and for practical purposes accomplishes the same things. But a full assignment will actually transfer ownership in a more complete manner.