Tank is one of core classes, essential in both defense and attack.
Strong sides:
Large amount of HP and strongest block in game make him hardest to kill.
Effective single-/multi-target disable skills.
Magic shield (more below).
Weak sides:
Small dps comparing to other melee classes
Some people find his gameplay boring. Everyone is busy killing stuff and you are just standing near, holding the shield. It's not really true, but if you are not ready for this: you'd better look for another class.
Name of the class may confuse some players with MMO RPG past, but he's not damage absorber, he can kill. In fact he can kill anyone in duel, but killing poor little things is not his main task.
So three main tactical skills:
Magic Shield. Yes, this starter skill will be your main skill right up to 30 lvl. It blocks all range (except Heavenly Flame where is my Sky Fire, Ariorh?, it just explodes on shield's surface and works as usually) and even melee attacks (but it's not easy even for an experienced player, so nobody expects you to do it). Holding shield doesn't give a lot of Class Points or experience and is not much fun either, but it saves the day.
I repeat front side of the shield blocks all attacks, including attacks of your allies. Back side however blocks nothing.
Mad skilz in case I'm not clear enough.
Previous feature leads to simple (yet a lot of new players fail at understanding of this) trick. Freeze assist, when there are two Ice Witches trying to freeze each other (and everyone around them) you can simply cover friendly Ice Witch with magic shield and help her to freeze enemy Witch (and save your frozen teammates as well).
Another piece of my art.
Schematic. Actual froze field is larger comparing to the shield.
Shield Strike. Great single target disabler. One of the things which give you dominance in duels. Just lmb enemies until they use Power Attack (smart run away though). If that's Berserker: use Shield Strike right after blocking his PA and he won't be able to block it (until he has a lot of Rage stacks or Paladin with Speed Powder nearby, but these are high levels). If that's everyone else: use Shield simultaneously with their PA, with your Health you can afford it. It's also good for skill interrupting (Freeze, Vortex, Resurrection and so on).
Battering Ram. Multi-target disable skill. Can be fatal for enemy team in combination with Freeze, Vortex, Heavenly Flame or simple rush. Just make sure there is no Inquisitor or Steam Hummer around, which may interrupt you. And of course your teammates must be near to make use of it. And even if there are... it's not a guaranty that they'll do anything, that's a problem of public games, so be ready for it.
Information about other skills you'll find in game and here. Just remember that Invulnerability protects you from everything but Chain, Gravity Mine and Fetters.
P.s. Tank doesn't have normal active skills until level 10, quite boring. I warned you.
I've been talking about magical shield of course. is there a way to avoid a slow effect with magical shield on? Because if i stay inside of this white circle around the witch i still have enough space to place shield between me and witch but I'm frozen anyway 