PAR stands for Parabolic Anodized Reflector.
What's that? Many halogen spot and flood lights use a parabolic (U-shaped) reflector to collect and reflect the light out the front of the bulb. Although this technique is generally not required in LED lamps, we continue to use the term as it's become an industry standard designation.
What does "MR" mean?
MR stands for Multi-faceted Reflector. Again, a technique for reflecting the maximum light out the front of a halogen lamp.
The term "MR" is usually only used for small lamps, 2 inches in diameter or less. These are called MR16s or MR11s.
What do the Numbers Mean? The number after PAR or MR indicates the diameter of the face of the lamp, expressed in 1/8ths of an inch.
In other words, an MR16 is 16 x 1/8 inches, or 2 inches in diameter.
A PAR20 is 20 eights-of-an-inch in diameter, or 2.5 inches.
A PAR38 is 38 eights-of-an-inch, so it is 4.75 inches in diameter.