Naraku (Nah-rah- KOO)
Naraku was born from the body of the human Onigumo Yatou and a bunch of demons. He is rather difficult to gauge, appearing in many forms, sometimes not appearing at all. His favorite strategy is trickery, and he is good at it. He has more shards of the sacred Jewel than anyone else, and often gives them out to other demons, in exchange for an assassination.
He currently resides in the body of a young lord. (Kagewaki) He has wavy black hair and dark eyes, which are rimmed by light blue. (This may or may not be eyeliner, but if you laugh at this, you are laughing at Sesshoumaru and Naraku, who will kick your ass easily and without a second thought, so don't so it)
He often wears a white baboon outfit, for reasons not yet explained. My personal theory is that he wears it because that outfit is the scariest damn thing I've ever seen.
On his back is a large burn mark in the shape of a spider, which appears on all of his incarnations.
When we first hear of Naraku, he is in the form of the human Onigumo.(spider demon) He was a thief who was badly burned in an accident we know nothing about. Kikyo took him in and treated him, as he was unable to move. He had to stay in a cave to avoid being captured for thievery. After weeks, perhaps even months, of seeing no one but Kikyo, and being unable to move at all, e fell madly in lust with her, willing to give up anything to have her love him back. This may have in pat been cabin fever and a bit of insanity. He called upon demons, who devoured his body. Onigumo and the demons formed a new entity, Naraku. Naraku is made of almost nothing but hate. He seems to have no ulterior motive. He continues to gather jewel shards, but this is simple for him, and he gives them away as freely as he comes across them, although he almost always gets them back. The heart of Onigumo beats within him, much to his displeasure, and it still seeks Kikyo, but Naraku has no interest in her, even going so far as to try to kill her a few times.
Most of the time, he just sits around his castle (for lack of better word) being moody and philosophizing, adding to the theory that he is insane.
Weaknesses: At some point in the month, he, like Inuyasha, becomes temporarily weakened. However, he can pick this time and uses the temporary separation from his demons to re-combine them in better, more powerful ways. He never appears in battle on his own, instead sending weak puppets which are easy to destroy. Once they are destroyed, he is blind as to what is happening.
Attacks: Naraku by himself is only a hanyou, and a rather weak one at that. However, he is made up of many many demons, and can create pretty much anything he wants on a whim. He created Kanna (void) and Kagura (wind). He is also very skilled in a form of puppetry, where he takes a tiny wooden doll and wraps a hair around it. It then forms into a puppet (kugatsu) of whatever form he chooses. They often resemble him, so it is difficult to tell who you are fighting, him or a puppet. He can create a miasma, or cloud of poisonous gases, and barriers (shouki) that used to work until Inuyasha found out how to cut through them.
Other than that, he is excessively strong, bit no stronger than pretty much everyone else in the series, so that is not really an issue.
He can also absorb other demons, and corrupt anyone with the large number of jewel shards that he has at immediate disposal.
He has a swarm of poison insects (saimyoushou) which always tell of his presence, but they also render Miroku's wind tunnel useless. Kagura is able to control wind, rendering the wind scar useless.

Kagura (KAH- goo- rah)
Kagura is one of the many incarnations of Naraku. She deals with the wind. She is controlled by Naraku, but wishes to be free from him. She wears a light pink patterned dress, and keeps her hair tied up, to keep it from blowing in the wind. She has a feather in her hair as well, which she can make bigger and use to escape. Her eyes are red, and pupil-less, and she wears green jade earrings.
Attacks: She has fans with which she can control wind. She can send out blades of wind, and other variations which she calls 'dances.' She can manipulate the bodies of the dead, and create huge hurricanes. She is also protected by half a million of Naraku's demons and some poisonous bees.
Weaknesses: She is completely controlled by Naraku, who holds her heart in his hand. She is also not that strong or fast, and so resorts to running away most of the time.
Kagura's job is to gather jewels shards for Naraku, and carry out whatever errands he doesn't feel like doing himself. She is resentful toward him, and seeks to betray him. She offered Sesshoumaru two of her jewel shards to destroy Naraku, he turned her down. The jewel shards were later taken back by Koga, from who they were stolen.
Kagura knows about Inuyasha's weakness and how he turns to human at the no moon, but she does not tell Naraku, because she wants them to destroy Naraku, I think. However, she is not able to be open about her rebellion, or refuse to attack them, because if she does Naraku will kill her.

Kanna (Kah Nah, there’s a definite gap there.)
Kanna is another incarnation of Naraku. She’s totally white, white hair, white dress, and a white mirror she carries everywhere. In the mirror can be reflected the future, past, or present. Kanna knows the future, but generally keeps her mouth shut. The mirror can also absorb souls, at least it could before it was hit with a purifying arrow. I don’t know if it still can.
More as I learn it.

Kohaku (Koh- HA- koo)
Kohaku is, or rather was, a demon slayer. After Naraku possessed him and forced him to kill everyone in his family, he himself was killed, only to be brought back to life with the power of a shard of the shikon no tama. Without that shard he would die, quickly. Once revived, he became really depressed with the memories, so Naraku took them away. In return, Kohaku must be his slave. Kohaku spends most of his time wishing he had his memories back, then when he gets them, begs Naraku to take tham away again.
He wears armor similar to Sango's only his is black and yellow. He has brown eyes which normally have no pupil, the international sign for zombiehood.
Attacks: He has a sickle on a chain, which he can throw with a good deal of accuracy. He is also relatively good with a sword and hand-to-hand combat.
Weaknesses: He's in an internal battle. Sometimes he isn't able to kill people because he knows it's wrong. Plus he's a zombie most of the time, which isn't good for him at all.

Lesser Demons
Or, as I like to call them, worm demons. They’re long and worm-shaped with nasty-looking heads, and are available by the millions. They’re the basic equivalent of foot soldiers: they go in, do what they can, and then get killed. They’re basicly there to slow somebody down, they attach by about a hundred at a time. They’re a problem for little kids, maybe, but even Shippo can take a bunch of them down.

Random Incarnations
Goshinki was third. He was able to read the minds of others, but lived only long enough to break Inuyasha's Tessaiga, an event that prompted Inuyasha's first transformation into his demon form. Sessoumaru used one of his fangs to create the Tokijen.
Kageromaru and Juromaru were vicious killers sent to fight Kouga, but they were destroyed by Inuyasha's Tessaiga. Juromaru was inside of Kageromeru, and controlled teh larger creature. He could go underground, causing sneak attacks.
Musou was made when Naraku tried to get rid of Onigumo’s heart. He was a melty-demon, meaning you could blow off whatever part of him you wanted and he would reform and be fine. He was in love with Kagome, who he thought was Kikyo, because he was basicly just Onigumo's heart. When he was created, he had no face, and killed something like a hundred guys trying to get a good one.
Akago, Naraku's baby, was the second attempt on Naraku's part to release his heart and this time he was successful. The baby was able to make others do what he wanted. The baby was cut in half during an attempt to read the heart of a priest. One half of the baby was eventually placed inside Moryoumaru.

(the rest of this I got from http://www.taiyoukai.com/naraku/)
Hakudoushi evolved from one half of the baby and assumed a child's form. He became master of the youkai horse Entei and was, of all the detachments, most like Naraku. Perhaps that is why he began plotting against his creator, a decision that led Naraku to revoke Hakudoushi's protection, which allowed Miroku to suck him into his Kazaana.
Byakuya is the latest of Naraku's incarnations. He specializes in illusions and appears to be rather laid-back and non-confrontational for the most part. He is something like Naraku's 'spy', since he first appears with the intention of observing Inuyasha using the Dragon-Scaled Tessaiga. Similar to Kagura, his mode of transportation is a paper crane that can be enlarged enough to fly.