Otherwise known as introverted. These type of people usually keep to themselves. For the most part, people who are withdrawn need prompting to do something, but once they feel comfortable will be able to hold a conversation for more than 20 secs. Usually never do things first and have very few "confidants".
On the flip side, they can be very trustworthy and usually end up being great people if you can get past their natural barriers.
"Mark is such a loner. He never talks"
"What are you talking about!? He talks to me. Turns out he's pretty deep. I guess you could call him withdrawn... but not a loner."