Stall:
The wing needs a minimum airspeed to create enough lift to keep the plane in the air. The wing is tilted upwards slightly (angle of attack) to create more lift. At low speed the angle of attack needs to be higher than at high speed. When a certain angle of attack is reached (normally 16-18 degrees) the wing stalls. It quits producing lift. When this happens, the plane will quit flying and start falling. This is what happens when you go too slow and hold full elevator to try to keep it flying.

Tip stall:
In some cases only one wing half stalls. One wing will drop because it lost lift (this happens fast). One situation that might create a tip stall is; you are flying very slowly, just above stall speed. You apply full aileron. The aileron that moves down will make the effective angle of attack higher, and stall the wing half. The other wing half will not stall because the aileron going up makes the effective angle of attack smaller.

To sum it up; tip stalls are stalls where only one wing half drops (instead of both wing halfs in a "normal" stall). Full aileron deflection at very low airspeed is one of the things that can cause a tip stall. Tapered-wing, highly aerobatic planes tip stall "easier" than trainers.