In simple terms, licenses and assignments are two ways to use content or authorize others to use your content.
In an assignment situation, a content owner assigns his rights. This means that the content owner is permanently giving away his content, or a portion of his content. An assignment is like a sale or a transfer of rights, whereas a license is comparable to a lease or a rental of rights. In a licensing situation, the content owner licenses a piece of his content, thereby temporarily permitting someone else to use it. For example, an author or photographer may give permission or a license to use his work on your website.
Using the words “assignment” or “license” may not by itself guarantee what types of rights are being granted. The wording used in a license agreement could be such that, in practice, it has a similar effect to an assignment. The key concept to understand is that rights may be exploited (usually in exchange for money) without necessarily being sold or permanently given away to someone else. For example, your library could license the rights to a database even though your library has no permanent or ownership rights in that database.
More on licensing for librarians: