Screenlets: Linux’s answer to Dashboard Widgets and MS Gadgets
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.