A bullet is the projectile that shoots out of a gun. It is not the entire piece of ammunition that you place into your gun (the entire piece is called a round or cartridge).
The type of gun you shoot(for example, 45ACP, 40SW, 38 Special) determines caliber of ammunition. Once you know the caliber, you need to choose the type of bullet you want to shoot. Knowing what you will use your gun for should help you determine bullet type.
Three common bullet types are:
Boat tail is a bullet feature rather than a bullet type. For instance, a full metal jacket bullet can be either traditional or boat tail.
Boat tail bullets have a tapered base that can be seen when the bullet is viewed from the side. Traditional bullets have a flat base.
The physics principle behind the design of boat tail ammunition is the same that is applied to a boat - the shape which is tapered at the "tail" or base helps reduce drag. This, in turn, provides more stability in flight and increases accuracy.