The ThinkPoint is IBMs name for the trackpoint that sits between the g,h an b keys of the keyboard. It is used as a pointing device and hence as an alternative to an external mouse or a touchpad. What sets it apart from trackpoints found in notebooks of other manufacturers is its tap-to-click feature, which means that you can tap the stick to do a left click instead of using the left mouse button.
The ThinkPoint uses the ps2 interface so it is supported by the standard psaux mouse input driver included in the linux kernel.
However, to make advanced use of it's features like configuring it's sensitivity there are different solutions available depending on the kernel you are using.