The registry could be described as  a database, located on your computer, that stores most configuration data (beginning from low-level configurations like which drivers for which graphics card are to be loaded up to program-specific configurations like which start page Internet Explorer should display).
The registry is hierarchically structured, meaning it is built like a tree.

If you want to have a look at your registry, you can do so by typing regedit at a command prompt. But be warned: do not change anything until you feel very comfortable with the registry contents!