A wall plate is a cover designed to enclose a box containing plug receptacles, switches, GFCI and more. It is designed to mount against the wall and cover the edges of the opening that contains the electrical box. The cover is attached directly to the wiring device and provides protection to the user from coming into contact with live electrical components.
A receptacle is a device designed to receive a plug and complete an electrical connection. The receptacle holds the contacts in place and provides electrical insulation to ground and the user. It is typically contained within a receptacle box and is covered with a wall plate.
Wall plates and receptacles are required to possess good insulating properties, be resistant to household chemicals and UV light and have a pleasing appearance.