A n00b is either a new nation, which does all sorts of things to ruin its reputation or a poor roleplayer, who never learned to RP. Noobs are infamous for godmodding, claiming to have n00ks, abysmal RPs or a combination of that all. In Sports roleplaying the term n00b also refers to a nation which does not participate (i.e.: RP) after signing up.
There is, in fact, some difference between newbies and n00bs. While "n00b" usually refers to a badly acting beginner, the more respected players often use the term "newbie" when referring to beginners that actually do roleplay and act maturely.