Kira is the Killer. Wielder of the Death Note. As such, it is Light, Misa, That guy Light "recruits" (who calls Light "god"), as well as the Shinigami/death gods (I watched it Sub'd, so I don't know what they called it in the dubbed version) and the company that recieved the Death Note when Light had his Memory Erased.
Matsuda is a young Japanese Police member working on the Kira case. he is very timid, but mans up in the last episode when he shoots Kira (Light)
Misa-misa is the 2nd Kira, getting her death note when a Death God gives his life to save her. Her goal was to find the original Kira, quickly falling hopelessly in love with Light. Light finds her annoying, even though she's a famous model head-over heels for him.
N or "Near" is L's successor. 12yrs old, he shares many characteristics with L, however he has white hair, and rather than loving sweets, plays with wooden Dolls that represent different people he is trying to find (Kira, him, Matsuda, etc)
in short, Light = Kira.