Definitions
Ailerons: Ailerons are control surfaces located on the trailing edge (the back edge) on the forward wing of the glider. With a basic slope glider, if you want to turn left, the aileron on the right wing goes down, and the aileron on the left wing goes up, thus banking the glider to the left, and vise versa. See Figure 1.
Ballast: Weight that is added to the glider to make it go faster and/or stabilize it in heavier winds. Ballast is placed right on or slightly forward of the center of gravity of the glider.
Computer Radio: The best type of radio you can get. Allows you to program the controls. For example, if you want your ailerons to be used as flaps to slow you down for a landing or thermalling, you can program the radio so your throttle stick will move both ailerons down (or up) instead of only being able to move the ailerons in opposite directions (requires one servo for each aileron). It also allows you to "mix" your controls (e.g. can give a little bit of rudder action with aileron action).
Covering: Covering an R/C glider is usually accomplished by using one of the popular covering materials such as Monokote or Ultracote. These are basically rolls of a paper-thin plastic-like material which sticks onto the glider when heated with an iron (not the type of iron you use for your clothes, a special iron you have to buy at the hobby shop). They also shrink and become pliable when they're heated. Being pliable allows you to stretch it over the curved parts of the plane and the shrinking makes it tight and gets rid of wrinkles.
Dihedral Angle: Umm... it's the angle of a wing. Kinda hard to explain. Easier to show it, so see Figure 2.
Elevator The control surface located along the horizontal stablizer at the back of the aircraft. It controls the pitch of the aircraft (up or down). See Figure 3.
Flatland Thermalling: The "other" way to fly an R/C glider. Usually accomplished by using a hi start (basically a really long rubber band) or winch (an electric puller type deal) to launch the plane into the air. Either of those can get a glider really high, really fast. Once they're in the air, the goal is to find thermals (rising currents of hot air, usually found above spots of land that the sun is heating up), get the glider even higher, and keep the glider up in the air for as long as possible. Done properly, with good enough conditions, gliders can be kept in the air for hours by using the thermals.
Fuselage: The "body" of the glider. See figure 4.
Pitcheron: Wings can pivot either in opposite directions (one pivots up, the other one down), or they can both pivot in the same direction which makes the wings act not only as ailerons to control the roll of the plane, but as elevators to control the angle of attack. With a pitcheron, there is no elevator in the tail.
Rudder: The rudder is the control surface that is located on the vertical stabilizer at the tail end of the aircraft. It's used to turn the glider left or right. See Figure 5.
Sheeting: For foam core wings, it's necessary to provide a protective sheet of wood for it. The wood is usually balsa or obeechi, and very thin (1/32"). The sheet is epoxied onto the wing and pressed (using the packaging foam core the wings come with) overnight.
Slope: A slope is a hill or cliff that can be used for slope gliders. In order for there to be lift for the gliders to stay in the air, there has to be wind hitting the face of the hill or cliff. The wind hits the face and is forced upwards. When the glider is thrown off the edge of the top of the hill or cliff, the wind that is going upwards keeps them aloft. Generally, the air currents behind the hill are quite turbulent and may even have downdrafts (air currents going down which usually equate to turning your glider into a pretty rock with wings that falls out of the sky).
Snap Roll: An aerobatic maneouver where the aircraft does a quick axial roll.
Speed Runs: A speed run is when you get your glider as high as you possibly can (or as high as you feel like), point the nose straight down to gain as much speed as possible, then level out to pass straight and fast in front of you (and as many spectators as you can), and then you usually point it straight up to get some major altitude again.
Wingeron: Wings pivot only in opposite directions (e.g. left wing pivots down and right wing pivots up). Used to control the roll of the aircraft (like ailerons).