There are many different bullet types and shapes such as flat-based, boat-tail, spitzer, very-low-drag, wadcutter, and round balls. The flight path and characteristics of each of these types vary slightly and therefore multiple drag functions have been created and optimized to match certain types of bullets. Here is a list of the available drag functions:
When a bullet manufacture creates a bullet they usually publish the BC or Ballistic Coefficient of the bullet on the package or on their website online. Sometimes they list several BC's for multiple Drag Functions. For example, Berger who makes a lot of VLD bullets often prints the G1 and G7 ballistic coefficient on the packaging. These Ballistic Coefficients are different so keep in mind if you are given a BC you have to match it with the correct Drag Function that it corresponds to. If in doubt, use G1, as it is the most popular by far.