The AH T-34 is modeled as useless against a Tiger in the game, but it is not impossible to disable and kill a Tiger. Try this offline: pull up to within a few dozen feet of the Tiger and make sure you are exactly 90 degrees to its side. Fire at the little plus sign (sorry, I forget what that symbol is called, kind of a modern Iron Cross). About 8 AP hits will blow up the Tiger. You can also take out the turret with one or two hits on that ring between the hull and the turret, but I am not sure what part of the ring is sensitive- I've done it once, but couldn't reproduce it. Online, however, these techniques are useless unless the Tiger driver is out to lunch or fallen asleep. While I have killed Tigers online with my T-34, it is always a Tiger that was previously damaged by someone else who then conviently died before I hit the Tiger.
Killing another T-34 with a T-34 is easy, even online, because of a bug in AH. Hit the exact junction between the top sloped front armor (glacis?) and the bottom sloped armor. In the real T-34 this junction is an enormously thick casting that is unlikely to be penetrated. In AH, however, I suspect it is modeled as the same thickness as the rest of the armor, but it is at 90 degrees instead of a glancing angle, so easier to penetrate. At medium to short ranges a single hit there with AP usually blows up the T-34. Again, you want to be at close to 90 degrees, i.e. the target is pointed directly at you.
Finally, although not relevant to this thread, killing a Panzer from a T-34 is easy: aim for the turret and make sure to hit it at 90 degrees (which, because it is about circular, means hit the center). Even relatively long range hits will take out the Panzer with one shot (or at least take out his turret).
