A tripod is a device with three stands (as opposed to the monopod, which only has one single stand). It is used to stabilize the camera when doing long exposures.
While the basic construction of every tripod is the same, there are different types of tripods that mainly differ in size, weight and load capacity. For example, there are small tabletop mini tripods which are very small and light. They are usually very cheap (~30$), but they cannot carry a big SLR camera with a lens attached. Cheaper tripods are built using plastic which is alot lighter than aluminium (used for the more expensive tripods like Manfrotto, Gitzo etc.), but also very unstable compared to the ones built with aluminium. There’s yet a even better material: Carbon - Lighter than aluminium and even more expensive, too.
Most “semi-pro” tripods have swapable heads that can be mounted on the tripod. There are mainly two different heads
The three-way head has three seperate locking controls for each movement axis. This allows precise adjustments on each direction at a time, but needs a little more time than using a ball head.

Manfrotto 808 RC4 three way head
The Ball Head has one single switch to open or lock the ball connected with the camera. This allows fast composition changes for portrait or landscape. It’s alot harder making small changes on the composition than the three-way head, though.
Manfrotto 486 RC 2 Ball Head
none.