Everyone has needed and used atleast one of the 1 liners in the famous awk1liners.txt "cheat sheet" text file. But not all of us would know exactly how the line you needed and used worked. Well here is a 3 part series on explaining them. I have not read it yet, but looks promising.
http://www.catonmat.net/blog/awk-one-liners-explained-part-one/
http://www.catonmat.net/blog/awk-one-liners-explained-part-two/
http://www.catonmat.net/blog/awk-one-liners-explained-part-three/