There are many different types of ammunition for airsoft guns. They range from low quality to very high quality and light weight to heavy weight.
The most inexpensive kind of BB for airsofting is .12 gram. These are very low quality compared to other weights. They may shoot at a higher velocity but at the expense of accuracy. They are sufficient for starter kits, simple electric guns and for people on a very tight budget. These should NEVER be used on higher quality airsoft guns because they may damage them and void the warranties.
The standard BB for airsofting is the .20 gram. Most airsoft guns are benchmarked with these bbs to test true FPS. This weight of bb will also give you the best ratio for speed and accuracy and work well with higher end AEGs and all other types of airsoft guns.
The veteran players choice of ammunition is the .25 gram weight. You loose speed and range but you gain accuracy. These are excellent for guns like the AfterMath Orphus, The UTG Model 4 Tactical, UTG Shadow Ops, Echo 1 USA M14 RIS and Echo 1 Vector Arms AK47 RIS that have the power to propel the heavy BBs.
.28 gram BBs are some of the heaviest BBs on the market and are extremely accurate but range and speed will suffer. These are excellent for upgraded AEGs and upgraded sniper rifles. These should never be used in starter airsoft guns because they are not designed to the extra weight