As an airsoft player, it is imperative that you use some kind of ammunition. Most guns work with 6mm BBs, as that is the standard size for most airsoft guns. Although the size is relatively uniform, there are many other factors that distinguish BBs from one another.
Even though 6mm is the main size, another size that you may come across is an 8mm BB. However, not many guns are compatible with this latter type, which is why you should stick to only looking for the 6mm variety. The main factor that distinguishes airsoft BBs is the weight. The most common weights are .12g and .20g. Each weight has its own advantages and disadvantages.
The cheaper of the two weights is .12g, because not as much material is required to make it. The drawback to them is they are less accurate and cannot go as far as .20g BBs. Due to their stronger build, .20g BBs are inherently more accurate.
In order to utilize .20g BBs, you need to have a relatively durable airsoft gun. If you try to use these kinds of BBs with weak airsoft guns, they are likely to break after only a few games.
Although .12g and .20g are the most common weights, there are many additional weights that exist, including .25g, .44g, and even .80g. Practically, you would not want to go above .44g, and even that is only used for the most powerful of sniper rifles. 
There are also special types of BBs out there, and these unique characteristics are independent of weight. For example, biogredable and tracer BBs are becoming more frequently used. Biodegradable ammunition is preferred because it is better for the environment, and tracers are used to be able to see where you are firing in the dark.
As you can see, you have many choices when it comes to purchasing BBs for airsoft. It is best to buy a lot of ammuntion at once, for two reasons. First, you save money when buying in bulk, and second, you save yourself the need to constantly repurchase BBs in the future.
