Gain is generally referenced based on two types, dBi and dBd. dBd is gain measured against an actual antenna (a dipole) while dBi is calculated against an antenna that only exists in theory, but can't actually be created. The formula for gain is dBd = dBi - 2.15 (or the converse of dBd + 2.15 = dBi). dBd is what I like to call "real gain" since it's something that you can actually get. dBi gain is only there to make the theory and formulas work out correctly.
dBi is often the gain that is quoted by folks trying to sell their antenna. Why you might ask? The answer to that is simple, it makes the gain numbers LOOK better while not actually being better. You know, like the EPA estimated MPG of that new car you're looking at, looks good, but you'll never get that much in normal driving.
To sum up, a 10 dBd gain antenna would be better than a 12 dBi antenna (it would have .15 extra gain). If they don't list what they measure the gain in, assume it's in dBi since you must assume that they're simply trying to make the numbers look better. If you're trying to compare antennas where some are listed in dBd and others are in dBi and still others do not list what the gain is reported in do the smart thing. Add 2.15 to those listed in dBd and compare away. They should now be on an even setting (yes, you may underestimate some that don't list what they report gain in, but it's really their own fault for not listing it).