The Fender bullets are specially designed to fit a tremolo system on a Fender guitar. The difference between the Fender bullets and any other guitar string is that their ball ends are not balls at all. They are shaped like bullets. The reason for this is 3 fold, first the bullet shape makes it easier to change strings on a tremolo bridge, a ball end string will usually get stuck in there and when it comes time to change it, it will be a pain to get it out.
The second reason is because its construction allows for more of the vibration to be transmitted to the bridge therefore giving you a warmer sound (complements of your instrument's wood) and a longer sustain.
Third: They stay in tune more consistently. A ball end will normally move when you depress the tremolo. It is unlikely that it will come to rest exactly on the same spot where it started, taking that string a little out of tune. With the bullets, this is not a problem since it is design to float up and down along the bridge's canal.
The strings themselves are Pure Nickel that give you that vintage bright sound.