Here's the skinny from OSHA on respirator classes.
The two main types are air-purifying respirators (APRs) and supplied-air respirators (SARs).
Air-purifying respirators can remove contaminants in the air that you breathe by filtering out particulates (e.g., dusts, metal fumes, mists, etc.). Other APRs purify air by adsorbing gases or vapors on a sorbent (adsorbing material) in a cartridge or canister. They are tight-fitting and are available in several forms:
mouth bit respirator (fits in the mouth and comes with a nose clip to hold nostrils closed - for escape purposes only)
quarter-mask (covering the nose and mouth),
half-face mask (covering the face from the nose to below the chin), or
full facepiece (covering the face from above the eyes to below the chin).
Respirators with a full facepiece also protect the eyes from exposure to irritating chemicals.