Ah, the Four Generals -- four exceedingly handsome men, supposedly the trusted advisors of Prince Endymion. Their names were Jadeite, Nephrite, Zoicite, and Kunzite. Unfortunately, they never really had a chance to prove themselves.
In the Silver Millenium, they were Endymion's four Generals. Each watched over a separate portion of the Earth, and all of them advised Endymion on matters of state. In a sense, they were his protectors, just as the inner senshi were the protectors of Sailormoon. However, they failed in their task.
Queen Beryl, having gained access to the powers of darkness, took over their minds. She convinced them that they needed to kill the people of the Moon, and that doing so would benefit Earth. The generals fell under her sway, and helped her raise a great army to ravage the Moon. However, Prince Endymion was not deceived. When the generals saw Beryl cut him down, they returned to their senses... but they were killed. It was too late.
They were reborn again, and immediately began searching for their prince... but Beryl had been waiting. She seized their minds with her dark powers, and made them her tools. They were the first enemies that Sailormoon and the inner senshi had to face.
You might have noticed that their names are all the names of gemstones. This theme is in keeping with the fact that Beryl's name is also a gemstone, and Queen Metallia's name refers to metal. Each general corresponds to a hunk of rock. These rocks seem to be their equivalent of the sailor crystals that the senshi have. Through these rocks, Beryl can control them, and Endymion can summon their spirits to advise him. Takeuchi-san identifies these stones in manga act 11, through Motoki's girlfriend Reika: (in the words of Alex Glover)
"Do you like jade? Jade refers to jadeite and nephrite. Jadeite comes in various colors, and it's called true jade. Nephrite is a stone of deep green. This pink spodumene stone is kunzite. Isn't it a light, pretty color? That pale purple stone called tanzanite is also known as blue zoisite. Tiffany's brought that one into the world."
Jadeite has short-cut blond hair and intense blue eyes. He is the Far Eastern division commander, and the first of the generals that Sailormoon faces. He likes to do his work through creations of dirt, shaped like people. Some of these creations are Naru's mother (lured people with jewelry sales; the jewelry was energy-draining), and the teacher at the Crystal Seminar (a reputedly good cram school; lured students such as Ami-chan and used their love of learning to drain them of energy). However, as Nephrite so kindly pointed out, these creations were fragile and easily defeated. This drove Jadeite to take matters into his own hands; he personally kidnapped several schoolgirls and a young priestess named Hino Rei, wishing to drain their energy at leisure. However, that last kidnapping led to his destruction; Sailormoon followed him into his pocket dimension and awakened Sailormars, who charred him to a crisp.
Nephrite has long, wavy brown hair and chestnut-brown eyes. He seems to have a friendly competition going with Jadeite; when Jadeite's dirt dolls fail to acquire sufficient energy, Nephrite points out that he, the North American general, could do a much better job. However, his rivalry with Jadeite is tempered by friendship; he was shocked and angered when Jadeite was killed. Part of Nephrite's motivation for seeking the ginzuishou was the hope that it would bring Jadeite back to life. Nephrite prefers to work through his shadow; he possesses other people from a distance and works through them. Straying away from that technique proved to be his downfall; when Sailormoon defeated his shadow for the second time, Nephrite appeared in person and Sailorjupiter disintegrated him with "Flower Hurricane."
Zoicite is very bishonen in manner, with soft green eyes. His wavy blond hair is tied back in a ponytail, and he is the general of the European division of Earth. He went about finding the ginzuishou by disguising himself as a woman. He posed as Professor Izono, student of the ginzuishou, and broadcast a message on national television begging people to find it. Zoicite-as-Izono threw Japan into a frenzy as people searched frantically for this mysterious jewel. (Usagi was not exempt; she tore through her dresser drawers in search of it.) Finally, when it was evident that no one knew where the crystal was, Zoicite sent out energy-draining waves from all the TV sets across Japan. This worked until Tuxedo Kamen roused Sailormoon, and she used the power of the ginzuishou to restore everyone's energy. Then Zoicite knew that she had the ginzuishou, and he went after Sailormoon himself. As he was about to kill her, Sailorvenus cut him to shreds.
Kunzite is the commander of the Middle Eastern division of the Earth. He is the leader of the four generals, and the last. His hair is long, white, and straight; his eyes are steel-grey, and a single diamond glitters coldly from one ear. He is calmer than the other generals, and more clear-headed; in fact, he was the only one to fight off Beryl's mind-control for even a little while. (He was aided by the power of the ginzuishou.) It's also clear that the generals are intrinsically good; they couldn't stand the presence of evil. When Kunzite approached too near Metallia, he found the air sickening and rank; he had to run away to breathe.
Anyway. Kunzite was the most effective of the four generals; while he could not kill Sailormoon, he was able to kill Tuxedo Kamen and kidnap his corpse. This action threw Sailormoon off balance emotionally, and gained time for Queen Beryl to act. Kunzite was later killed by all four senshi acting together. (Sailor Planet Attack!)
Kunzite seems more attached to Zoicite than to the rest of the generals, which lends itself to the rumor of a yaoi relationship between them. However, it must be noted such a relationship has never been concretely proven. The closeness that they share could be attributed to other things than being lovers.
For more in-depth analyses of the four generals: