Product Overview — Tinkerforge: "There are four different types of hardware components:
Bricks: Stackable microcontroller boards for sensing and controlling.
Master Extensions: Boards that extend the communication interfaces of a Master Brick.
Bricklets: Non-stackable sensor/actor boards that extend the features of a Brick.
Power Supplies: Boards to power a Stack of Bricks, plugged below the Stack.
See the tutorial for an explanation of how everything works together."