Thief is a DPS-focused profession that kills the enemies fast, hinders them with traps and uses stealth to attack from shadows, not to forget poisoning them to death. Since thief deals lots of damage but gets also killed easily if hit, thief relies on evading enemies' attacks and thus requires good movement skills to survive.
As such, thief has probably always been one of the popular DPS-classes and especially lately it has become very strong to the point of being considered one of the strongest DPS classes. (Though, once again, a bad thief still does way worse damage than good player on other classes.)
With Heart of Thorns, staff-spinning thieves became a common sight when thief got the "Daredevil" elite specialization. As very melee-damage focused specialization, daredevil shines in the melee-range but not so much outside it. Being a melee-specialist, daredevil has horrible DPS uptime at any bosses that require people to take distance to survive the mechanics and usually evading the said mechanics ends up as a lethally risky yet best choice.
Though thief ish best known for single-target DPS, it also does very strong AoE damage, especially the power staff-variant and with Invigoration Precision's "heals 15% of crit damage done", a staff thief is almost invulnerable in lots of AoE-content.
With Path of Fire, thief goes ranged with "Deadeye" elite specialization. It marks the enemies and shoots them from far away with a rifle, dealing extremely heavy hits...but with high initiative cost, of course. Also, a rifle-using deadeye needs to kneel, staying still to deal the highest damage and thus it's far better in sniping enemies in PVP/WvW-content than in lots of PVE-content. Rifle's also a ranged weapon and like all ranged weapons, does rather low constant DPS despite super-heavy bursts.
Kitty tested deadeye in PVP during demo and she managed to kill 4 enemies attacking her allies in mere moments by sniping them from afar so as long as deadeye stays hidden yet in range, it makes for an excellent sniper.