I would say:
newb = someone who is new to something, and thus, doesn't understand it very well yet.
noob = someone who is not new to something, but still doesn't understand it very well yet.
Of course, something that someone says on the internet could mean anything really. That person may use noob as "a person I dislike" or "a person who didn't get the joke" or "someone young."
Usually, it is just used as a simple insult used to ruffle someones feathers, and has no meaning at all.