Depends how you build her. I'll list all build paths that i know of and try to summarize what they do.
Warrior/Runeglaive, Luden's Echo, Rylai's, Tank-items
Is tanky. Besides that, cc with R and Rylai's. Run after people and slow them. Q based.
Warrior, BotrK, Trinity?, Tank-items
A bit less tanly than the previous example, but more dmg and Movespeed. Based on E dmg and TF procs. Bruiser style
Runeglaive, Luden's Echo, Gunblade?, Ap-items
Full assassin style. Burst people - run away.
You have to keep in mind, that you can build a lot of stuff on her. Like getting Trinity Force, Hextech Gunblade or Luden's Echo for damage and then buy any Tank-items you want.
MY preferred build is "AP oneshot":
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Runeglaive, Luden's Echo, Nashor's Tooth (procs twice on E), Rabadons, Voidstaff.
Then exchanging Runeglaive for Lichbane. Last item Tank-item or another Ap-item. No boots needed, since you have Lichbane and Luden's.
E has a 190% Ap-ratio with Lichbane, Nashor's and Spellblade-mastery. If you get ahead with this build you can literally 1shot people with E. Risky though.