The "tail" end of the bullet and the "base" are generally synonomous. Bullets can be flat based, with the longitudinal surface ending at 90 degree angle with the bullet base. With "Boat tail" bullets the longitudinal surface stops short of the base and changes angle towards the bullets central axis before making a 90 degree angle with the base. In other words, the base is narrower than a flat version. If you lay such a bullet on a flat surface and look down on it, it is the shape of a boat. In my experience boat tail bullets are more accurate than flat based bullets when shooting at long ranges.
There are other specialty versions such as hollow base, gas check, etc. Good reloading manuals such as those published by Sierra, Hornady, Speer and Nosler, discuss bullets and their characteristics in great detail.