Screenlets are small owner-drawn applications (written in Python) that can be described as "the virtual representation of things lying/standing around on your desk". Sticky notes, clocks, rulers, ... the possibilities are endless.
References:
Web: https://packages.debian.org/jessie/screenlets
File: Debian -- Details of package screenlets in jessie.pdf

Features:
Real applications, no HTML-"widgets"
Easy to use, easy to develop
Full compositing support
Works with any composited X desktop (compiz, xfce4, ...)
Works also on non-composited desktop
Included ability to apply themes (SVG, PNG or mixed)
Fully scalable when using SVGs
Embedded drag&drop-support
Automated storing of options (using ini or GConf)
Controllable through customizable D-Bus service
Can be used together with compiz' widget-plugin to create a
Dashboard-like feature as seen on OS X
Uses Cairo and GTK2 for drawing and windowing

Installation:
$ sudo nano /etc/apt/sources.list
$ sudo apt-get update
$ sudo apt-cache search screenlets
$ sudo apt-cache show screenlets
$ sudo apt-get install screenlets
Note: Desktop XFCE also adds Screenlets when you install all its complementary software. XFCE also installs Wbar, which is similar to Cairo-dock and docky.
References:
Guides: XFCE, Cairo-dock
Topic: Docky
Web: https://packages.debian.org/jessie/x11/docky
Topic: Wbar
Web: https://packages.debian.org/jessie/x11/wbar

References:
Topic: These guides contain installation of Desktop programs similar or equivalent to Screenlets.
Guides:
Conky
Fluxbox
Off topic: These guides are interestingly related to the current topic and Desktop customization:
Guides:
Cairo-dock
Compiz-Fusion
Gnome Shell Extensions
Gnome Desktop Icons, Themes and Cursors
Desktop Extensions
Multiplatform Desktop Applications
Google Chrome