The acronym MERV stands for "Minimum Efficiency Reporting Value". The MERV rating is the standard method for comparing the efficiency of an air filter. The higher the MERV rating, the better the filter is at removing particles from the air.
The MERV scale ranges from 1 (least efficient) to 16 (most efficient), and measures a filter's ability remove particles from 3 to 10 microns in size. Filters with higher ratings not only remove more particles from the air, they also remove smaller particles. A typical fiberglass furnace filter might be rated from 1 to 4 on the MERV scale. A HEPA filter would be rated a 16. (However, the HEPA requirements are substantially more stringent than a MERV rating of 16 -- therefore, filters with a MERV rating of 16 are not necessarily HEPA-compliant.)