Legend

hvc	False color image, derived by making a combination of the visible and infrared channels of the NOAA satellites. Image color represents temperature - not representative of real 'earth color'
ir	Infrared: This is a direct channel of the NOAA satellites, which is always present, unlike the other channel, which switches from visible to another infrared band during day/night
mcir	Map Color IR: If visible light is not available, MCIR uses an internal map in the decoding software to distinguish between continent and water, and still be able to render a color image. The other information is taken only from the IR channel
msa	Multispectral analysis, sophisticated name to refer to yet another channel combination method (like HVC) which uses both NOAA channels to render a false color image.
mb	Indicates probability of thunder storms. Areas of low-temperature clouds are enhanced to mark possible strong storms.
no	Infrared, but extreme low temperature are rendered in strong colors. I particularly like this mode, as it indicates possible areas of dangerously active cloud formations better than any other mode. Black, and even worse - white areas, are at -60 C or less indicating very strong possibility of hail and other problems.
vis	Visible range. These pictures are logically only available during daytime imagery.
za	Infrared, with non-linear representation, enhancing the temperature differences in cloud formations. This permits finding the storm centers.

Modes ending in 'm'

If a filename has a mode listed in the table above, but has an extra 'm' at the end (like 'irm', 'mcirm', etc.), then the images are actually mosaics: two or more images, obtained from consecutive satellite passes, glued together into one (larger) image.

From where I live, I rarely have the opportunity to obtain two passes of sufficient quality to merit the combination into a mosaic. Still, when it does happen, the result is normally worth the extra download time!