Whilst it is definitely not an Apple invention, Dashboard Widgets surely added another way for Mac users to summon and hide small utilities with a click of a button. Widgets have become popular that even Google and Microsoft added it to their arsenal. Linux, otoh, will not be left far behind so it created Screenlets. I think it is best described by watching a video.
Installation is quite simple – add the repository, do a "wget" and install. That is it! For the full details, check out their website.