Grit refers to the size of the particles, and as with sandpaper, the smaller the number, the larger the particles are. 60/90 ?- which means a range of particle sizes between 60 and 90 grit -- is the coarsest grit generally used in tumbling, though 46/70 is available. 500F or 600F is generally used for the prepolish step, although some people use 800F or even finer for prepolish. (The "F" means "and finer".)
Mesh is a little more precise, referring to the screen through which a particular grit must pass. The term mesh is more commonly used for abrasive sizes 1000 and higher. For practical purposes, however, grit and mesh can be considered the same thing: 1000 grit = 1000 mesh.
A micron is 1/1000 of a millimeter. About the only time you'll come across this measurement is with certain polishes, like Linde A and Linde B. In the case of microns, the larger the number, the larger the particles -- the inverse of how the terms grit and mesh work. Therefore, 120 grit equals about 100 microns, whereas 220 grit equals 70 microns and 600 grit equals 25 microns.
Silicon carbide is the basic abrasive used in tumbling. It is very hard ?- Mohs 9+ -- and each time it breaks, a new sharp edge is exposed. This makes it long-lasting and quite effective.
Boron carbide serves much the same function as silicon carbide, except that it is slightly harder. This makes it the abrasive of choice for tumble-polishing corundum, although it is both expensive and messy to use.
Aluminum oxide is a synthetic material most often used as a prepolish and polish, although you can get it in coarser grades. The advantage of aluminum oxide (or alumina) is that it breaks down to a more rounded edge than does silicon carbide, which is what you want in the finer stages of processing a batch. Aluminum oxide is Mohs 9.
CPP tumble polish is an effective and economical aluminum oxide-based polish for barrel tumblers. If you just want one polish for most stones, this is the one to go with.
Cerium oxide is a natural polishing compound particularly useful for materials that are between Mohs 5 and 7, like glass and quartz. It is more effective in a rotary tumbler than in a vibratory one, as the intense vibrations of the latter break the grains down very quickly.
Tin oxide is 99.9% chemically pure and finely graded in the submicron size. It's recommended as a final polish in flat lapping and for metal/stone combinations.
Chrome oxide is the medium of choice for getting a good polish on jade, lapis lazuli, rhodonite, peridot, and other colored stones.
Pro Polish polishes a wider variety of gemstones than any other material, working like Tin Oxide but at half the price. It is composed of finely-graded aluminum oxide in the 1/2 micron range. Polishing Tip: works best in a thin slurry.
Vibra-Dry is a proprietary prepolish and polish compound specially designed for vibratory tumblers. It requires no liquids, which makes inspection and cleanup very easy, and is excellent for soft, fragile, or difficult-to-polish materials. Vibra-Dry is also reusable. Processing cycles are somewhat longer than with polishes that use a liquid medium.
Sapphire powder -- also sold as Linde A (0.3 micron) and Linde B (0.05 micron) ?- is a synthetic polish manufactured from aluminum oxide. It has a hardness of Mohs 9, and is therefore most useful for polishing very hard and/or difficult materials (but not corundum).
Diamond powder is the hardest abrasive available ?- Mohs 10. It's also probably the most expensive, and is generally sold by the caret! However, if you need to get a good polish on corundum, diamond powder is probably the most effective way to go.