There are two basic types of SMDs used in automotive lighting, the 5050 SMD and the 3528 SMD.  The names given to these LEDs is derived from their size.  The 5050 SMD measures 50mm x 50mm and the 3528 SMD measures 35mm x 28mm.  The differences do not stop at the size.  The 3528 SMD uses a single LED chip while the 5050 SMD is a 3 chip design.  This can be helpful for the novice to indentify the SMD properly.  By observing the images, you can see that there are 3 contacts on the side of a 5050 SMD and 1 contact on the side of the 3528 SMD.

5050 SMD		3528 SMD

The light output and the current requirements are also different. Theoretically, the light output of a 5050 SMD is 3 times the light output of a 3528, but the actual output is a little lower.  Power consumption of a 5050 SMD is

The benefits of these SMDs are pretty apparent.  The 5050 SMD is the obvious choice for brightness (light output), while the 3528 SMD is a smaller package.  The 5050 is a clear choice in most cases, but when space is a concern, the smaller 3528 allows for easier fit.