SMD LEDs are surface mounted devices which are placed directly onto a circuit board and then soldered using a reflow soldering machine or hand soldered in some situations. Generally a SMD LED light will have a lot of smaller LEDs on a circuit board attached to a heat sink or fixture.
A COB type LED or chip-on-board is a small array of LED dies which sit directly onto a circuit board and then with a single phosphor plate directly above them. This method makes the surface area required a lot smaller and the use of single optics, including reflectors easier.