Combos are poorly documented in-game (if at all) but they are awesome and useful. (This seems common for ArenaNet.) They are like LotRO’s fellowship maneuvers but simpler, less powerful, more mobile, and more varied. (This post adds “thought” to the previous “hey, these exist.”)
Let me digress immediately to note that fellowship maneuvers are how you can tell if you are in a really good fellowship. If your group nails its first three for long straights, you know this is going to be the best PUG of the month. They are why you want burglars in your group, and the bosses that are immune to fellowship maneuvers can be That One Boss because they take away one of your best tools for damage and recovery, knock out a key class feature, and eliminate a player skill factor that separates the great from the good.
Combos were mentioned during development, perhaps not under that name, and they do not seem to have had a lot of attention since. Remember something about shooting arrows through a wall of fire, then they become flaming arrows? This is that, although I’m not clear how often “shoot through” works versus “shoot from/into.” Combos are like getting free buffs and spells just for standing in the right spot.
GW2 expands “don’t stand in the fire” to “don’t stand in the red circles.” Combos add “do stand in the white circles.” It seems like half the weapon skills are fields or finishers. The link has a full explanation of what that means, but the easy version is “use your finisher skills in the white circles.”
I had several strong recommendations to use Evasive Arcana, but since I had been playing a scepter/focus elementalist, I had only one field available out of twenty skills. The unadvertised feature of Evasive Arcana is that all four effects are blast finishers, meaning that if you end your dodge in a field, you become a fountain of buffs. Staff elementalists get six fields.
Think of your elementalist as a healer for a moment. Two of those six fields are for healing and regeneration, and then you grant healing, regeneration, and condition-curing when you switch to water for your healing. Earth has a time-delayed blast finisher, so you could start that, switch to water, cast a healing field on the finisher, roll into it, and hit Arcane Wave. You just comboed three AE heals, plus several other heal and buff effects, and you have the base earth attack under that.
If someone else stacked a field on yours, you might have just triple-triggered that one, too. [Update: commenter says "no"] And then everyone in your healing field gets to use their finishers, say the rangers who start getting stacks of regeneration from their shortbow attacks.
The next time you are in a big group fight, like a giant or dragon event, watch for people dropping fields on large clumps of characters. Last month, I might have thought those were griefers trying to annoy people by putting fire and lightning walls in their faces. And then the giant got 25 stacks of vulnerability in two seconds, plus several minutes’ worth of burning, and the players started stacking might and swiftness.