Every type of skill that can inflict a DoT skill adds an additional count up to a max of 5. Stacking these increases your actual damage on the enemy. Most people use the skill Mist Eruption when the DoT counter is at 5, because that's when such a powerful skill will hit the hardest.
These are the skills that cause DoT. The first 4 are your main skills, the last 3 have CD so don't rely on them.