ok, you said a lot of info and you've got it, so there may be spoilers here, but i'll try not to include them.
Death note summary:
it's about a brainiac high school kid named Light Yagami, who is extremely bored with life and how rotten the world is. then a similarly bored shinigami (death god) named Ryuuk decides to drop his death note in the human world just for kicks. Light picks it up. he reads the rules: "a human whose name is written in this notebook shall die." of course he doesn't believe it at first but can't resist trying. after a couple of tries, he discovers the book is real and decides he will use it to rid the world of evil and create a utopia with him as the god of the new world. when tons of criminals just suddenly drop dead in a span of five days, the international police convenes and asks L, the best detective in the world whose real name, face and whereabouts are unknown, to solve the case of Kira (Light's name to the public). thus a battle of wits begins between the two, with whoever gets found first shall die.
What's a Death Note?
Death Note is a notebook owned by the shinigami (death gods). these notes have rules (not written in exact words):
1) a human whose name is written in the Death Note shall die.
2) the writer should also know the face of the person he wants to kill, so the note cannot confuse him with another person sharing the same name.
3) if the cause of death is written within 40 seconds of writing the name, it shall happen.
4) if the cause of death is not specified, the person will just die from a heart attack in 40 seconds.
5) if the writer manages to include the cause of death, details of the death can be written in the next 6 minutes and 40 seconds.
these notes are used by the shinigami to lengthen their lives. shinigami eyes can see the names and lifespans of humans, so that's very convenient for such a purpose. they kill a human using the notee and whatever remains of that human's lifespan will be added to the shinigami's lifespan, thus lengthening his life. if they don't kill people they will die. (there are tons of other rules regarding the Death Note, all of which are revealed throughout the show. i remember most of them but these are the basic rules and therefore all we really need to know)
**Shinigami Eyes
the eyes play an important part in Death Note say they have to be mentioned. shinigami eyes can see the names and lifespans of all humans. an owner of a Death Note can make 'the deal' with the shinigami who once owned the note. the deal is that the owner can have the shinigami eyes at the cost of one-half of his life. so if you make the deal and have 100 years of your life, you'll have 50 years left. when Ryuuk offers this deal to Light, he wisely refuses. Misa makes this deal twice, once with Rem and again with Ryuuk thus significantly shortening her life. Mikami makes the deal with Ryuuk.
Characters:
Light Yagami/Kira - the protagonist bad guy of the show. the protagonist does NOT have to be the good guy (like Macbeth), but he does have to be the main character, the one where the story revolves, and that's Light. he is a 17-year old highschool student, incredibly smart with a great sense of justice but bored. when he finds the Death Note, he uses it to fulfill his vision of creating a utopia, a world with no crime and sin, with him as god and his word as law.
L - the antagonist, who in this case is the good guy, since the technical definition of antagonist is one who opposes the main character/protagonist. he is regarded as the world's greatest detective, and the only person in the world who has the power to mobilize the world's police. his face, name and whereabouts are unknown to everyone, and he can only be contacted through his assistant, Watari, whose face and real name are also unknown. his other aliases, Erald Coil and Danuve are also the top 2 and 3 best detectives in the world, but the people don't know it's just one person. L has a lot of weird characteristics: he sits with his knees up to his chin (he says sitting normally decreases his reasoning by 40%), eats nothing but sweets, holds things delicately and hates to wear shoes. his appearance covers his great skills as a detective. he is an orphan by the way, and multi-national.
Misa Amane/Second Kira - self-appointed girlfriend of Light. Kira killed the murderer of her parents which sparked her admiration and love for him. she was given a note by the shinigami Rem, when another shinigami named Jealous saved Misa's life and died doing it. Misa uses the notebook to find the real Kira, disguising herself as the Second Kira. she is a famous actress and pop star in Japan, but not very smart so Light keeps her in a tight leash. you can say she has deluded herself into thinking that Light does love her, but the only reason Light heeps her is because she has the shinigami eyes.
Ryuuk - the shinigami who drops his note which is picked up by Light. he doesn't really give a crap about what goes on, and says he is neither on Kira's side or L's side. he doesn't tell Light immediately important things about the note, which usually irks Light. he does things for the sake of apples, which he loves.
Teru Mikami - a worshipper of Kira, in every aspect of the word (he calls Kira "god"). he is tasked by Light to act as Kira when he was driven to a corner, a role Mikami relishes. he is a little unstable, saying "Eliminate" when he writes names on the note, and more extreme than Light, wanting to kill people who are lazy and those who committed crimes in the past but are now repentant, a fact that also alarmed Light. he is a prosecutor and in his childhood was bullied for his strong sense of justice and righteousness. he also had great deduction skills.
Nate River/Near - the first in line to succeed L. he shares some similarities with L, like the sitting position and likes to play with things like toy robots, just how L played with his food. he is smart, withdrawn and logical, and as opposed to Mello, he got the support of the US government to track down Kira and is the leader of the SPK (Special Provision for Kira). proof of his brains is his 3 years' worth of independent investigations before the founding of SPK.
Mihael Keehl/Mello - my favorite character :) second in line to succeed L. he did not get along with Near because he can't take being second best, so when Near asked him to work together to track Kira he turned him down and left the orphanage at 15. he ended up in a Mafia gang in LA, using them as his henchmen to find Kira. he is just as smart and logical as Near, but his emotions usually get the better of him. he shares L's love for sweets as Mello is usually seen eating chocolate. he is considered a criminal as well because he uses illegal means to get what he wants (like kidnapping), but his help is invaluable to the SPK, because he manages to find out more about the note and relays his knowledge to them in exchange for his only picture in the world.
Kiyomi Takada - Light's college classmate and 'girlfriend' (at the same time as Misa). when Light joins the hunt for Kira with L, they grew distant. after graduating she gets a job as a newscaster in a Japanese network NHN and eventually gets chosen by Mikami as a spokesperson for Kira, without knowing her past connection with Light/Kira. Light calls her proud and smart, but Near disagrees with the latter description, saying she is only studious but not smart. like Misa she gets used by Light to further his plans, saying she will be "the goddess of the new world".
these are the most important characters in the entire show. other supporting characters are also important but there are too many so refer to Wikipedia for descriptions.
Japanese Investigation Team:
Soichiro Yagami (Light's dad)
Tota Matsuda (the show's comic relief)
Shuichi Aizawa (afro guy who first suspected Light as Kira after 'that' episode)
Kanzo Mogi (the quiet buff guy)
Hirokazu Ukita (killed by Misa)
Hideki Ide (joined the team only when Light became L)
Watari (real name: Quillsh Wammy; L's assistant and owner of Wammy House, the orphanage where L, Near and Mello came from)
SPK members:
Commander Lester (sort of like Near's right hand man)
Halle Lidner (Mello's connection to the SPK, at least in the manga)
Giovanni
Yostuba group:
i don't know most of the people here, but the only member you really need to know is Kyosuke Higuchi, the person acting as Kira when Light purposely lost his memory to remove suspicion from him. Eventually he is caught by L and Light, and is killed by Light when his memories returned.
bad person: Light, but he is the protagonist
good person: everyone against Light like L, Near, Mello, the SPK and those in the Japanese investigation team, but they are the antagonist because they oppose our main character.
NOT COPIED FROM WIKIPEDIA!!!