N95 - Filters at least 95% of airborne particles. Not resistant to oil.
Surgical N95 - A NIOSH-approved N95 respirator that has also been cleared by the Food and Drug Administration (FDA) as a surgical mask.
N99 - Filters at least 99% of airborne particles. Not resistant to oil.
N100 - Filters at least 99.97% of airborne particles. Not resistant to oil.
R95 - Filters at least 95% of airborne particles. Somewhat resistant to oil.
P95 - Filters at least 95% of airborne particles. Strongly resistant to oil.
P99 - Filters at least 99% of airborne particles. Strongly resistant to oil.
P100 - Filters at least 99.97% of airborne particles. Strongly resistant to oil.
http://www.cdc.gov/niosh/npptl/topics/respirators/disp_part/default.html (http://www.cdc.gov/niosh/npptl/topics/respirators/disp_part/default.html)