Elementalist is a light armor profession that attunes to elements to burn the enemies with fire, summon a soothing rain, strikes them with lightnings or to bleed them with duststorms. Thus it's well capable of taking damage dealer or healer role in various stuffs by using various specialization combinations. Weapons-wise, elementalist builds have way more freedom of what weapons to use compared most of the other DPS-builds since most weapon combos do various tasks decently, some being especially good for certain aspects of elementalism. For utilities, elementalist can summons elementals, conjure elemental weapons, control the arcane energy or cast glyphs for various elemental effects. As light armor user with lowest healthpool, elementalist requires good movement and smart gameplay to survive.
Currently various DPS-elementalist builds are among the most popular for various stuffs these days and people are glad to have a DPS elementalist or a couple in their squads, especially in endgame PVE-contents.
Meanwhile, though healer elementalist excels in damage mitigation and supporting the squad with auras, it lacks the offensive boons of a druid and thus healer elementalists are very rarely used in raids and fractals despite its strong heal-support capabilities.
Overloading empowered elementalist a lot, not to forget the powerful warhorn skills. For a long time now, DPS-tempests have been considered one of the highest DPS-classes with all their burn and fast air attacks. And though healer-tempest isn't considered a raid build ('cause druid offensive support), it does work quite well for fractals as its heals and auras can carry even weaker teams through lots of stuff. Healer tempest is also widely used in WvW and PVP for its damage mitigation skills.
On the Path of Fire, elementalist starts weaving elements together for more powerful spells, faster attuning and main-hand sword as the new weapon of the Weaver.
In short, Weaver can use 2 elements at once, with primary element on skill slots 1-2 and secondary element on skill slots 4-5. Skill slot 3 is reserved for a combination skill weaved from the 2 elements present. When weaver attunes to new element, the new element becomes the primary element and old primary element becomes the secondary elements. This allows weaver to have whole 2 complete elemental skills sets to use at short cooldown. Weaver can also double-attune to be fully attuned to one element with ol' good weapon skill 3 to use.
Here's a condiweaving example: the weaver starts the fight fully attuned to fire, quickly using fire skills 2 and 3. Then weaver attunes to earth which takes over skill slots 1-2. Weaver presses 5-4-3-2 to use fire skills 4-5, mixed fire/earth skill 3 and earth skill 2. Then weaver attunes to fire, making earth the secondary element and presses 5-4-2 to use earth skills 4-5 and fire skill 2. Then, weaver starts swapping between earth and fire to use the skills as they come off the cooldown.
Kitty's not entirely sure if the weaver utility skills and 2 complete elemental skill sets to use at once outperforms tempest's shouts, overloads and warhorn skills. Guess we'll see when Kitty starts benchmarking.