From wikipedia (which has everything you ever wanted to know about the show, including spoilers):
"The Office is presented as a mockumentary. The primary vehicle for the show is that a camera crew has decided to film Dunder Mifflin and its employees, seemingly around the clock. The presence of the camera is acknowledged by the characters, especially Michael Scott, who enthusiastically participates in the filming. Others, for example Jan Levinson, are frequently annoyed or uncomfortable at its presence. The main action of the show is supplemented with talking-head interviews or "confessionals", with the characters speaking one on one with the camera crew about the day's events. Sometimes two characters share an interview, speaking with each other and the camera at the same time. This occurs most notably with Jim and Pam, or, occasionally, Oscar and Kevin or Kelly and Ryan, and once, Michael and Toby. Dwight frequently interrupts Michael's interviews, as he is often standing off-screen next to Michael as the interview begins."