Newb is short for newbie. This can also be spelled as noob, n00b, nub and other leet combinations.
Why is it bad to be a newb? Because it completely sucks. You are looked down upon as being non-knowledgeable. Why do they alienate you because of this? No one really likes newbs. And if they do it's to take advantage of them for example in videogames, newbies would get owned by leet veterans or vets, pros, eXperts etc.
According to www.dictionary.com newbie is defined as n. Slang
One that is new to something, especially a novice at using computer technology or the Internet.
The way it is used, is usually concerning computer technology; videogames and programming.
Josh Munce defines newbie as: a newbie refers to someone that is new to a social group, way of life, method of personal action, or other social groups. Generally meant to give the new-comer a friendly hard time, although it is often used as an insult in gaming. "Did you see that guy? What a total newbie." (credit: taken from www.LangMaker.com)
Of course, you may ask, "Wtf. Everyone is a newbie at one point or another" Yeah, I know it's not really a question except for the 'What' in 'What the fuck." But I do understand what you mean. Everyone is a newb once at everything. But when you are promoted from a novice to a "leet xpert" you are to be respected basicly because of the increase in skill and experience required. Being a newb is easy; You just have to not know what you're doing. It is much more difficult to be a 'total pwnage user"
Anyway, that explained nearly nothing. But I will add more rants about newbies and newbism.