I keep hearing about "tip stall".
What is it and what causes it?

From the [link=http://www.rcuniverse.com/community/glossary.cfm?letter=T&pref1set=all&cirkus=0&sort=1 &key=]RCU Glossary[/link]:

Ken

As the plane flys slower and slower, the airflow over the wing changes from totally perpendicular to more spanwise. This happens more at the tip of the wing than at the root. Air spills off the tip of the wing, causing loss of lift at the tip. In a severe situation, the tip will stall from lack of airflow. The wing tip will drop, and the stall will progress inward to the root. A tip stall can be quick and violent, and happens with no warning. It often rolls the plane on its side or even turns it completely over. Since this usually happens when the plane is low and slow, there's little chance for recovery.

Dr.1

As the wing moves through the air there is a rotor effect off the tips of the wing. At cruising speed these rotors are relatively small and fall off and behind the wing harmlessly, well they reduce efficiency. As the plane slows this rotor gets a little bigger and starts hitting the wing, as it does this that portion of the wing stops creating lift starts at the tip and moves in . at some point the wing stops creating lift and one will drop off before the other which is a tip stall.
Some designs are more prone to this than others. a thick chord wing. short wing span and long front to back has more of a tendency to tip stall than a long wing with a short chord ie thermal gliders. Though there are many factors that influence it like wing thickness airfoil shape and overall weight of the model.

If you have a model that is prone to tip stall like a CAP 232 or a heavy warbird you avoid the tip stall buy keeping your speed up. Use as much runway as you can at least on the first couple flights on take off. Come in hot when you land until you are sure of the planes safe landing speed. You avoid it with experiance in knowing how to recongize what is happening and reacte correcty without over correcting. Usually what I see happen is someone does catch the tip stall but over corrects and snaps the model the other way.

Typically occurs on landing approach.
Avoid tipstall by keeping the planes nose pointed slightly downward throughout the entire landing approach. When the plane gets about 1-2' off the ground ever so slightly pull the nose up and slowly increase elevator as the plane descends. When the plan is about 3" off the ground, hold it there with the elevator... Try to keep it 3" off the ground as long as possible until it gently sits and rolls to a stop. Use the throttle instead of the elevator to control rate of descent on approach. Avoid pulling the nose up on landing approach, especially when the plane is slowing way down and 5-10' high,, thats when you see most tipstalls.

Can also occur in flight if you let the plane get too slow. Most frequently occurs as you make a turn to go downwind. As you are rounding the corner and lining up downwind, if you are a little too slow,,,, as you add elevator in the turn to maintain altitude you will do what the guys below said. The plane will suddenly drop a wing and sprial into the ground. Once a plane tipstalls about the only way to recover is to drop the nose and dive to gain airspeed. Throttle only makes it worse. If you have the altitude, once you gain airspeed to get the wing flying again you can ease the throttle in and you are back to normal flight. A lot of time its not recoverable unless you are pretty high, like 50' or even better 100' high.

Some planes, like trainers are almost impossible to tipstall. Planes like p51's, Edge 540, Extra 330, and the infamous Giles are really easy to tipstall if you slow them down too much unless you really REALLY know what you are doing.

It is countered in some AC by washout. The wing is warped slightly so that the wing has a lesser angle of attack at it's tip than at the root. The wing will stall at the root, and then the stall will procede towards the tip allowing for better control through the stall. Piper Cub is a an example of this.

Constant chord wings like those found on trainers tend to stall at the wings root first. Highly tapered wings tend to stall at the wingtips first. The aerobatic maneuver known as a "snap roll" is a tip stall done on purpose. With enough elevator, a stall can happen at any speed.
When you do a first flight on a new plane, get high up and do a tight loop with the stick all the way back. If this causes it to snap roll, land the plane very carefully or let an experienced flyer land the plane. Then reduce elevator throws or make the plane more nose heavy.
If you can not make the plane snap roll even with full back stick, it probably won't snap on you while landing either.
You can also slow way down at a safe altitude and explore your new plane's stall charactoristics. Most trainers will just drop their noses and dive until they have airspeed again. Some planes, like scale Cubs, may go into a spin as they stall. A spin is nothing but a snap roll in the down direction and it is caused by a tip stall.

Both good comments. I agree 100%. Washout really can help cut down on stall problems and you can do it with the monokote on a built up wing. You just twist the wingtip downward a little towards the leading edge and then tighten up wrinkles in the monokote while your helper holds the twist in the wing. Trick is to get both wings right,,, and just alike.

Doesnt take but just a tiny bit... Im oversimplifying it a bit. I would do it the first time with someone who has done it before. Once you have seen it done its a snap, no pun intended!

Caviot; trainers have it built in, I have never seen one that needed it unless the wing is warped.

My first crash was due to a tip stall. I was 10 feet off the ground and and dropped the throttle to half, put the wings perpendicular to the ground and tried to do a quick 180 using the elevator. I got about halfway through the turn and next thing I know the plane was cartwheeling through the dirt. I never saw it coming and it happened so fast I didn't even have time to blink.

My ol' Cessna 150 had a mechanical stall warning system. It was a plastic party horn with the noise making reed reversed so that it would make noise when you sucked on it. It was attached to the stall port with a tube so when the angle of attack reached a certian point the horn would sound. I used to take it up and do what I called wheep stalls. You'd slow the plane as you increased A of A until the horn just began to wheep, then I'd pull and release the control yoke to make the horn go wheep wheep wheep. Ah to be that young and stupid again. I guess my point is(other than stroll down memory lane) that the stall port is near the wing root and while the root was approaching stall, washout allowed for good control at the ailerons.

As stated by others here, tip stall is simply that. The tip of the wing stalls prior to the remainder of the wing and usually results in loss of control or almost such. 1/1 scales are usually designed with either significant outer wing washout or a symmetrical tip section with a much more lifting section at the root, usually progressive from the root, along the wing span, to the tip. this places the inner wing sections at a higher angle of attack thus normally initiating the stall at the root section then outward from the wingroot.

While most here relate the problem to landing approaches, I have witnessed many heavy (relatively speaking of model airplanes) "War-Birds" bite the dust ON TAKE-OFF because the RC pilot tried to fly it like a Q-500 or some easy taildragger such as a 4-Star 40,60, etc. The snap or near-snap just after lift-off results from a tip stall condition because the pilot does not induce or maintain proper rudder control rather than simply using aileron.
The model is allowed to leave the earth too early with the tail still down. model is not really yet "flying". It is lifted off at a high angle of attack. It rolls left, the pilot quickly uses aileron -- rather than rudder -- to correct this. The left down-aileron completes the stall requirements for the left wing and so another nice model rolls over into the ground. the pilot has no clue to his problem, which is simply poor pilotage.
Had the pilot used rudder to maintain a straight flight out, plus maintaining a climb angle of not more than 10-12* he would not have had a bent-up machine.

Actually the same applies to landings. Use rudder coordination with the ailerons on approach. During the round-out phase try to stabilize the landing by maintaining slight rudder downwind and some aileron to hold the wing slightly down into the wind.
Definitely coordinate the rudder for any large aileron inputs. Rudder can prevent adverse yaw which is also a contributor to the tip-stall, be it takeoff or landing, or a high speed turn around a pylon.

Angle-of-attack (AOA) is the only factor that a pilot controls to make a wing stall. Everything else is simply prevention of exceeding critical AOA.

Edit; Typo