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