Somebody who devotes their life to one specific activity because they have absolutely nothing better to do in their spare time. One such example is somebody who plays video games more than 12 hours a day because they have nothing better to do. No activity is exempt from this though, and video games can't be singled out, this applies to sports, outdoor activities, etc...
no_lifer: You have no life. All you do is play video games.
other_guy: No, I have other things I can do. I just choose to play video games.
no_lifer: Well what about me. All I have to do with my life is play football? I'm not a no lifer. I have friends.
other_guy: Actually, you don't have a life. Just having friends and spending your time playing sports doesn't mean you automatically have a life.
Someone who updates twitter / facebook / their personal blog (WTF, who owns these? we don't care what you think!) practically every moment, detailing EVERY SINGLE event that happened in their day.
So you're trying to prove to us that you have a life...but you honestly don't. We don't care what you're doing.
A: -twitters- MM, I'm hungry.
A: -twitters- YUM going to starbucks now!
A: -facebooks- Going to my boyfriends now!
A: -twitters later- Giving my bf a bj! GTG!
Someone who attempts to ruin other peoples lives, for a living (i.e. no life), because it is the only way they can gain emotional power or control in their own life.
People who sue or nag you; just because they can (no lifer).
Parents/Gaurdians who take away everything (friends, cell phones, allowances) so that they can feel like they have authority over your life.