Wow, had to check in on this thead, having so many replies for such a simple topic. Tip stall is simply the outer portion of the wing stalling before the inboard portion, which causes you to lose roll control (because your ailerons are in the stalled portion of the wing). Washout or a different root vs. tip airfoil section is used to ensure the inboard portion of the wing stalls first- wing stalls, you still have aileron (roll) control, the airplane naturally pitches down a little, the wing starts flying again- that's it.

Maybe this simple explanation was included in the thread already, but really it's too much to wade through for such a simple topic.

Yes, at the very tippy tip of the wing, there is no lift as the wingtip vortices are slipping off, but a "tip stall" is not meant to refer to the outer most .01% of the wing. And yes, sometimes the term can be misapplied, such as when someone is banking hard at a slow speed; the wing stalls and the plane falls or spins in in the bank direction- not a tip stall, just a stall in a bad attitude.

K.I.S.S.