What is assignment?
Within the context of copyright law, an assignment is where a copyright owner transfers all of his exclusive rights (except for moral rights, which are not transferable) in a protected work to another party. While an assignment can be limited in terms of its time or geography, if it includes a transfer of less than all of the rights, then it is actually a license (where only some of the rights are granted). A larger distinction between an assignment and a license is that with an assignment, even the copyright owner himself can no longer exercise copyright rights (because they have all been transferred to the other party), whereas a copyright owner may be able to continue exercising his own rights after licensing those rights to a third party (whether he can or not depends on the terms of the license agreement).
An assignment must be in writing and signed by the copyright owner (whereas a license does not have to be written or signed, although it should be).