You can find a full definition of copyright on the U.S. Copyright office website. The following is a very short description.
Copyright is a legal concept that gives the creator of an original work exclusive rights to it. The copyright holder has the right to choose if anyone else can use, adapt or resell their work and have the right to be credited for that work.
Only the copyright holder of a work may do the following:
Remember the creator of a work retains copyright to it even if they don’t expressly tell you so.