Kenta: An ANBU is a high-ranked level ninja of the Naruto world. If you've read the first five books, you know Naruto and Co. start the series as Genin, Junior Ninja. The Third Hokage and Iruka explain, but I'll do it anyway.
The level of skill of a ninja is determined by rank. To rise in rank, a series of tests must be taken.
The ranks are, in order:
Ninja Acadamy Student - Genin - Chunin - Jonin - Special Jonin - ANBU - Kage (The Hokage ("Ho" in Japanese is a word for fire) is the 'Fire Shadow' of the land of Fire, Hidden Leaf Village)
Missions are ranked based on the skill level required to complete said missions. D and C-ranked missions are given to Genin, C and B-ranked missions to Chunin, A and B-ranked missions given to Jonin. However, a skill level higher than Jonin, which would be ANBU (Since Special Jonin is pretty much the same as Jonin) is mostly tasked with even more dangerous missions, ranked S.
Yikes, just looking at this I feel like a total Narutard...lol oh well. Shows how much I like the series.