Without getting too scientific, SMD (surface mounted diodes) LED downlights consist of an LED module which is essentially a panel consisting of many small LED chips mounted on the face, acting as a decentralised (spread) light source.
On the contrary, COB (chip on board) LED downlights consist of a larger single LED chip mounted directly on a circuit board (or substrate). It then uses reflectors inside the light to throw the light downwards, acting as more of a central light source in the middle of the downlight.
Do they look different? How can I identify the types?
Yes, you can spot the difference. SMD LED downlights generally have a larger surface area on the face and have a larger frosted cover over the LEDs to diffuse the light, just like the Mercator Optica shown in the image above. COB LED downlights do not have a frosted cover over the LEDs and look more like traditional halogen downlights in their appearance.