Boosting - taking a high level toon and grouping it with low-level toons. Using the high level toon to kill, thus "boosting" the amount of xp a group can gain in a short amount of time as a high level toon can kill many more mobs a lot faster than a full group of same-level toons. People usually boost in dungeons as elites are higher XP, there's no competition and mobs are pretty close together for easy gathering, but there are some outdoor spots people use as well.

It's pretty boring work.