A tray is a shallow container designed for carrying things.
Trays are flat, but with raised edges to stop things from sliding off of them. They are often rectangular, and may or may not have handles with which to carry them.
A butler's tray has a deeper surround, handles on the short sides (usually cut into the surround) so that it can be easily carried, and folding legs. It is used to carry drinks, and can be used as a side table.