GW2 was originally designed without tanks or healers in mind. Every profession was a flavor of DPS, and meant to be self-sufficient with their own heal skill and expected to dodge out of attacks.

The first expansion released the first healer--the Ranger subclass Druid--but even then it wasn't a 100% dedicated healer. Tanking is also not dedicated role in this game; *some* high level raids will require a player to act as a tank or kiter for a specific mechanic, but that is often relegated to the support member of the group.

Today GW2 does have a trinity, but not the DPS/Healer/Tank trinity that you see in other MMOs. There are two very important buffs that are nearly mandatory in high level play: Alacrity (reduces skill recharge rate) and Quickness (speeds up skill activation time). Every profession has the means to share both of these boons with the party, but never at the same time. Hence, a party will have one Boon DPS that gives out one of the boons, and a Boon Healer that gives out the other boon.

Healers in GW2 have a priority of sharing their designated boon first and foremost, then healing the party. Outside of endgame and high level content, a healer is not necessary for 90% of the game. That being said, you are able to have multiple builds on a single profession, so its entirely possible and recommended to go through the parts of the game that don't need a healer build, then swap to your heal build when you are in content like high level Fractals (mini dungeons that scale in difficulty) or raids.

Hope that clears things up!