They are typically rectangular and yellow, like their physical counterpart, but most applications support other colours and more elaborate designs.
For the Macintosh, the Stickies application was developed by Jens Alfke and included in System 7.5, released in 1994.
[1] A number of applications have duplicated the functionality of Stickies on other platforms.
macOS has its own built-in desktop note functionality with the Stickies application and, from Mac OS X Tiger through macOS Mojave, with Dashboard, an application that has notes and other desktop widgets.
On Linux, desktop notes have existed for a long period of time.