Guardian is a versatile heavy armor profession with a light-related skill set to block hits, protect the allies and punish the enemies with divine justice. Guardian uses symbols to damage foes and grant beneficial effects to allies and it can also summon spirit weapons for help to smite the enemies or shelter allies. Guardian has the lowest health pool of heavy armor classes, but compensates with lots of blocks and damage mitigation skills.
Guardian has mainly been a DPS-class for some time now, with power dragonhunter as the most popular build due to the damage it does while other builds have been left to dust by most despite their effectiveness.
Healer-guard is very protective with strong heals but lacks the offensive support of druid. Meanwhile condiguardian is also a good alternative to power DH with better DPS uptime, especially thanks to spirit weapon revamp and idiotproof rotation.
With Heart of Thorns, guardian got a DPS elite spec "Dragonhunter", which allows guardian to use traps and longbow to assist in the hunt. As dragonhunter, virtues gain physical aspects, like Shield of Courage to block attacks in front of the dragonhunter and Spear of Justice to burn and cause the enemy to take more damage and also to pull them to the dragonhunter.
Longbow never became popular in PVE-community due to longer attack times and the most powerful longbow skills requiring the dragonhunter to stand still for the activation time. As dragonhunter's skill set and traits are power-focused, Scepter+Torch/Greatsword ended up as dragonhunter's weapons of choice for now. Though Kitty does personally favour longbow for AoE-heavy content.
With Path of Fire, guardian gets another elite spec "Firebrand" which uses axe and tomes to rain fire on the enemies and to provide the allies with various boons and heals, quickness being the most noticeable of them. Firebrand also adds mantras as guardian's utility skills.
Tomes replace virtues and they have 5 or 8 pages to use before they need to reload, Tome of Justice contains strong damaging and burning spells, Tome of Resolve has spells to rejuvenate allies and Tome of Courage empowers the allies with various boons.
Kitty personally finds firebrand as a strong condi and healer spec. During her tests in PVP demo weekend, she saw very strong burn stacks but also noticed that it's difficult, nigh impossible to keep the burnstacks stable. Healer-tanking-firebrand can easily grant 100% quickness upkeep to allies while healing them and tanking effectively.