Dashboard
The dashboard is one of the functionalities offered with Tiger, the latest version of MacOS X. The Dashboard is an extra screen that appears and disappears at will over the normal screen.
It is used to store small applications that are used occasionally such as a calculator, post-its etc. or informative/entertaining ones such as the weather forecast, following a commercial flight in real time, without cluttering the main screen.
These applications are called widgets and are actually an assembly of HTML, CSS and Javascript (and occasionally, compiled language). Apple provides several of them by default, but anybody can write them and there are already enthusiasts' sites burgeoning with original widgets.