A derelict can is normally a space craft or small orbital station that is no longer active or manned. Derelicts should not be confused with wrecks. A wreck is a ship that has been virtually destroyed and all that remains is wreckage to be salvaged.
Derelicts come in many shapes and sizes. They are classified in a Range from A1 to C10. A1 being the smallest of the type and C10 being large structures like battleships or large orbital stations.
Derelicts can be located in almost any place where humanity has traveled, although you will require the correct scanning equipment to locate them.