The combos consist of Initiators and Finishers. An initiator is a skill that causes an area of effect in the world, such as the Elementalist ‘Flamewall’ which creates a line of flame that damages enemies. This opens up the field for a Finisher Combo.
A Finisher is a Blast, Leap, Projectile or Whirl attack, such as the Warrior’s ‘Cyclone Axe’. If a Warrior stands in an Elementalist’s ‘Flamewall’ and uses ‘Cyclone Axe’ it will create a combo, and that sends fiery bolts in an area around him. Your skill descriptions will state whether the skill is a combo field or finisher or neither.
Combos prove to be very helpful in PvP, WvW, and PvE. Some professions will be able to do combos by themselves, such as the Thief, who can drop a Smoke Field and then use a Blast Finisher to stealth their entire party. However, the large number of combos available are through two characters of different professions working in tandem.