Two prevailing theories on thermal types:
1. Bubble thermals
2. Column thermals
Be aware that both drift downwind.
And clouds tend to mark thermals as below.
Sometimes in cloud streets.
1. The direction of turn in the thermal is determined by the first glider in the thermal.
2. If at same altitude as another glider in a thermal, position yourself on opposite side of circle
3. Do not thermal directly below another glider
Finding a Core
The core of the thermal contains the best lift. On the fringes you will find less lift, or even sink. See below for a top-down cross section of a thermal. The dark green represents the best lift. Red is sink.
Here's an image showing a 270 correction if you hit the fringe of a thermal and want to snoop around for its core.