What is a CFM?
CFMs, or Cubic Feet per Minute, is a measure of volumetric flow. In the world of fans it's used to tell how powerful a fan is. The higher the CFM, the more air that fan pushes or pulls through. Depending on the application, there are specific requirements for how effective a fan must be.
How Much CFM Do I Need?
For a major appliance such as a gas range, Abt recommends 1 CFM per 100 BTUs. For example: a GE $1,000 gas range has five burners, totaling 47,000 BTUs. That implies you'll need a exhaust vent with a fan rated at 470 CFM. That's assuming, though, that you have all five burners on at the same time. In the bathroom, you might want to upgrade your fan if your stomach doesn't process burritos well.
It might sound simple, but make sure your fan fits! Many high powered bathroom fans are designed to slide into 2x6 or bigger ceiling joists, so if you don't have the space you might have to settle for a lower power model. Also consider when buying a range hood for your kitchen that the space above your range might not match exactly. If you have a 30" free standing range surrounded by cabinets, the upper cabinets on the sides might have end panels that eat into your 30" of available space. Also be careful to note where the fan exhausts (it's not always straight up, sometimes those stacks are cosmetic) and make sure you have a spot to run your exhaust outside.