There are three main forms of lift - thermal, ridge and wave. Thermals are rising columns (actually bubbles) of air created by the sun's heat. Ridge lift is created when moderate or strong winds blow over a hill or ridge, and get deflected upwards. Wave lift is created by large hills and mountains, and is on a much larger scale than ridge lift. Wave lift can take you much higher than thermal or ridge lift, ten's of thousands of feet high. In some places it can go into the stratosphere.

Below is my description of each form of lift. However there is a better version complete with diagrams at the BGA website's Soaring Page.

Thermals are created by the heat from the sun. During the day the sun heats up the ground. This creates bubbles of air that are warmer than the surrounding air. When these break away from the ground they rise up, creating lift. Thermals are shaped like a torus, with the 'column' of rising air in the middle, and sinking air on the outside. Most of the time, when the air in the thermal gets high enough, it cools enough for the moisture in it to condense and form a cumulus cloud at the top. Looking for these clouds (also have been described as 'fair weather clouds' on certain forecasts) is one way of finding these thermals.

However, when the pressure is high enough an inversion can limit how high the thermals can go, and often prevents cloud from forming. They are called 'blue thermals'. This makes finding them trickier, as they are then completely invisible (and usually weaker, too). Note: an inversion is where the air above is warmer than the air below. On a blue thermal day, if the air below is warmed up enough so that it warms up more than the air above, the inversion breaks, and conditions suddenly get very good for a while.

To make use of a thermal, you have to circle in it, and stay in it. You have to make sure that the centre of the circle is the center of the thermal to make the best use of it. On a good day, a thermal can take you all the way to 4,800 feet above ground level, the limit at Lasham Gliding Club (large aircraft fly above this level). In many places around the world, thermals will take you much higher than this on a good day.

Ridge lift is created when wind blows over a hill, or against a ridge. Because the hill or ridge is sticking out of the ground it deflects the air upwards. The air being deflected upwards can be used, if strong enough, to gain height, hence 'ridge lift'. This method of lift can only be used if the wind is strong enough, and only works to a limited height above the ground.

To use ridge lift you position the glider at one end of the hill / ridge (so that the hill / ridge is across wind). You fly the glider so that it moves along the hill / ridge, where the lift is, as slowly as possible. When you reach the other end, you turn the glider so that you move back along the hill / ridge again. The glider is aimed so that the nose is aimed upwind enough to stop the glider flying forward into wind, but not letting it drift down wind.

Wave lift can be thought of in a similar way to waves on water. A hill or mountain will deflect air upwards and then it will come down again. Then, further downwind the air will bounce up again. What results is a wave of air rising upwards. At the tops (crests) of each wave you tend to get a lenticular-shaped cloud if it goes high enough. Soaring where the air is rising is much like soaring in ridge lift, except it can go an awful lot higher. Good wave lift can easily take a glider to more than 20,000 feet, and the record is somewhere in the 40,000's.

Like water waves, air waves can interfere with each other. If you have two mountains positioned one downwind of the other, if two wave crests downwind occur in the same place, constructive interference results in higher and stonger lift in that place. Conversely, destructive interference (a crest and trough together) can cancel out the wave effect altogether.

I've also heard of thermal induced wave aswell. This can happen if the air above a thermal cloud is moving in a different direction to the air underneath (the wind shear must be at least 30 degrees). The air can get deflected over the cloud, as if it were a hill, causing a wave. As a result, it can sometimes be possible to get weak wave lift over flat ground. This effect may only be likely on weakly thermic days, as otherwise it tends to be too turbulent to occur.