Here is where you can know a bit about each group and what is expected of them, or qualities.
Kira is the alter-ego of Light Yagami. The name "Kira", comes from the Japanese for "killer".
Kira can be used to describe any one person who uses the Death Note, as well as the entity worshipped by some members of the public.
The Japanese Task Force, (commonly referred to as the Task Force, or Kira Investigation Team) is the primary organization searching for Kira. The Task Force was created by L after he deduces that Kira is in Japan, and is the only organization within the country that is searching for Kira. The existence of the organization is not made public, and the names and personal information of all of the members' is kept secret. Most of these members are police men, FBI agents and so forth, but they are specifically hand picked by L himself.
The Special Provision for Kira, usually abbreviated SPK, is a group of "elite" anti-Kira Central Intelligence Agency (CIA) and Federal Bureau of Investigation (FBI) members headed by Near. Membership of the group remains small to preserve secrecy and the group decides to work separately from the Japanese Task Force. These are even more exclusive, and Near knows them personally one way or another, based on their past cases, or their infamy in crime fighting. They interact with the Task Force, but do not work for them. Because of the egos of Near and L, there is usually a mediator, being their "Wataris" or second in commands.
"The Mafia" is a west coast criminal organization in Los Angeles allying with Mello. The group intends to posses a Death Note and thereby gain power. Mello intends to beat Near and catch Kira, but needs a Death Note to do so. He joins the Mafia which is led by Rodd Ross, who incidentally wants to increase the Mafia's influence. By allying together, Mello profits from the Mafia's resources and Ross profits from Mello's intellect. Having proved his usefulness by finding a rival gang that "even Kira couldn't catch", Mello becomes a primary member, with Ross taking Mello's advice. Mello's closest friend, Matt, also from Wammy's House followed him, and is also a part of the Mafia, along with Mello.
Shinigami, or Gods of Death, are a race of extra-dimensional beings who survive by killing humans to extend their own lives. Shinigami in this series are not responsible for every death that occurs; people will eventually die regardless of whether or not the Shinigami pay attention to them, but a Shinigami can end their lives sooner than intended to extend their own lifespan. They are not usually evil (though some take joy in killing others and do it for selfish reasons), some are mercy killers, cutting the life span when feeling that they should not feel such pain. Each Shinigami is different and their motives are their own, so it's up to each Shinigami whether or not they kill or not.
Citizens are people living within the Kanto Region. Usually they are just bystanders to the whole thing between Kira and L. But even these seemingly normal people can become something more, and something vital in the storyline.
Criminals are people who constantly commit crimes. Kira normaly eradicates the criminals in prisons, but some are small time, petty criminals that have not been caught. However, time will catch up with them all, and perhaps, Kira "right their wrongs" eventually.
Spies are the individuals that live more on the dangerous side. These people spy on whichever side is more convenient to them. Their motives are different based on the individual, but what is clear that death is a high chance to happen.
Natural Kiras are lucky individuals that are born with a special gift. It is said that you can get Shinigami eyes, through being chosen, if a Shinigami falls in love/ finds a child worthy it is given this gift, or if a promise is made. A myth says that eyeballs rather than a Death Note was dropped to Earth. These people have a shorter lifetime, usually die young because of this inadvertent sacrifice.
Wammy's House (located in Winchester, England) is one of many orphanages established by Quillsh Wammy (aka Watari). Founded after World War II, Wammy's House began as a training facility for children with exceptional intelligence and skills. After the success of the detective L, the motive of the school was to raise children to be the successor to the original L. None of the children refer to (or know) each other by their real names, instead using aliases. In addition to these protective aliases, notable individuals who graduate from Wammy's House may be assigned a 'letter' by Watari, now by his successor, Roger Ruvie. Until the holder of that letter dies, they are understood to be recognized by the current L and as all that have connections to Wammy's House as having significant potential to positively affect the world, and work for its change. The letters closest to L (M and N) are often direct candidates for L's succession
A Kira follower is someone who agrees with Kira's aims and believes in what Kira what Kira is doing. Some people believe Kira is righting the world of it's wrongs, and believe Kira is justice. Notable followers of Kira are Misa Amane, Teru Mikami and Kiyomi Takada.
People under this group have sworn to uphold the law. While their job is dangerous to begin with, it seems that with Kira now rampant, it's a safer job, for criminals find their deaths. Still, there is a question whether or not it's a good thing, and they are divided by the question "what is justice?"
The Federal Bureau of Investigation (FBI) is a governmental agency belonging to the United States Department of Justice that serves as both a federal criminal investigative body and an internal intelligence agency (counterintelligence). They have taken interested in the case in Japan because of it's eerily similar case called the Los Angeles BB cases.
These are special individuals that work in the entertainment business. They are famous in radio, movies, or television. These people usually are held above others, but ever since the whole situation in the Sakura TV station, they do realize that there is a chance Kira is among them, and they are wary of their surroundings.