Vulkan He'stan is the Forgefather of the Salamanders chapter of Space Marines, once the Captain of the Fourth Company. He should not be confused with Vulkan, the Primarch of the Salamanders; He'stan used to have a different first name, but he gave it up for Vulkan's when he became the Forgefather.
Part of his duty as Forgefather is to seek the artifacts that Vulkan made and scattered throughout the galaxy. In particular, four remain to be found: the Engine of Woe, the Obsidian Chariot, the Unbound Flame, and the Song of Entropy. His task would be made slightly easier if anyone alive knew what these were, but apparently, Vulkan only left the names. Vulkan spent some time in a casket inscribed with the words Unbound Flame, but he did not make the casket, it is on Nocturne, not lost, and its nature is known to both the Salamanders and the Ultramarines, rather than a total mystery.
Fortunately, He'stan wears three of the recovered relics to help him on his way: the Gauntlet of the Forge (which has a built-in heavy flamer), Kesare's Mantle (which gives him a 3++ invulnerable save), and the Spear of Vulkan, a master-crafted relic blade.
He also speaks Dark Eldar fluently enough that a Haemonculus was surprised by it.
In addition to having the statline of a Brother-Captain (with a 2+/3++ instead of a 3+ armor save) and all his tricked-out wargear, Vulkan He'stan confers a great advantage on the army that he leads: all meltaguns, combi-meltas, and multi-meltas become master-crafted (in addition to Salamanders Chapter Tactics, of course, so a model with both has two master-crafted weapons). For this reason (not to mention being one of the Salamanders, and therefore one of the few genuinely good guys in the Warhammer 40,000 universe), he is a popular choice to lead Space Marine armies.
A common strategy is to field a list full of meltas, combi-meltas, flamers, and combi-flamers packed into drop pods when using him. This is because using a melta becomes a lot more reliable when using Vulkan He'Stan, meaning you can count on the meltas to at least hit (and we all know what happens when a Meltagun hits). Also of note is that he makes stock Dreadnoughts incredibly efficient when used in a drop pod. If you upgrade the dreadnought to Venerable, you literally have a 2.78% chance of missing the target. Great for when you absolutely have to kill that pesky Land Raider.
Another thing of note is that despite his army-buffing prowess (the biggest reason most people take him), he's got great prowess of his own. While having "only" the stat-line of a Captain, he's actually a steal in terms of points. A Salamanders captain with a 2+/3++, heavy flamer, digital weapons, and a (master crafted for free, since salamanders) Relic Blade costs 165 points (and remember that a Captain, heck, anything not a Sternguard, Termie or The Legion of the Damned, can't normally take a Heavy Flamer!). This means master crafting every melta in your army costs only 25 points; plus, when he's your Warlord, on top of master-crafting your meltas, he gets FnP(5+).
Also, his special rules also master-crafts the bolter part of combi-meltas, which is pretty awesome, especially with Sternguards (confirmed by page 5 of the current main rulebook FAQ). Furthermore, he should master-craft his own bolt pistol with his Chapter Tactics (Salamanders).