SK's are an offensive tank class, they have defensive (hate, shield masteries, etc) skills as well as offensive skills the other tanks do not have, such as lightning strike, drain, freezing strike. I don't play one so that's all I have for you ;)
SK's also have cubics, which use D crystals to summon little crystals that float around and aid you in combat. An example of some cubic effects are draining life from opponents and giving it to you, or debuffing your enemies during combat.
According to one of my clanmates SK's are fairly effective at soloing compared to other tank classes. The Vampiric Cubic (which drains life from enemies) when used in conjunction with the Drain Health spell that all Dark Elves have cuts downtime quite a bit. DEs also have a high offensive potential, so even a tank class is capable of dealing quite a bit of damage.
The SK is a tank, so you get all the tanking good stuff (hate aura, ultimate defence etc) for solo/group survivability, however DE tanks have the lowest HP of all races, so some ppl feel safer having a human knight tanking.
The SK can solo with little downtime due to having 2 life drain abilities and a vampiric cubic that acts as a 3rd passive life drain (doesn't fire off all the time though).
There are a few ways to play the SK, making them suitable for many different situations. Ppl play them as tanks, as DDs and as fighter/mages. Since you have 4 "nukes" (2 drains, freezing strike and lightning strike) using a weapon with high m.atk will make those nukes viable for PvE/PvP.
Cubics need D crystals, so having cubics out while grinding gets expensive. Just having the vampiric cubic out is fine solo.