ASLer is a term denoting a subspecies of humans. While they retain human characteristics like emotion, they fall short in many categories necessary to consider them full blown humans.
ASLers operate similar to cavemen, or, more modernly, wolves. They have a pack mentality, which assists them in achieving their main goals: pissing off other people in chat rooms and all over the internet, and having torrid sessions of badly-spelled cybersex.
The pack mentality assists these goals by making it impossible to simply pick off one ASLer with one of the strongest weapons against them, polysyllabic words. When one ASLer is attacked, the rest of the pack will use tactics like flooding the room or bailing en masse to save their asses.
The ASLer does not only exist in chat rooms; most n00bs in online games are, in fact ASLers. However, this is not a safe assumption; even the best players were once n00bs, so they are obviously all not of the subspecies.
The best offense against an ASLer is to simply not pay attention to them, although that will soon become harder and harder, as every airheaded cheerleader and down-on-his-luck skateboarder learns to use a computer.