Rainmeter is a free, open-source application for Windows PCs. It is a platform that enables skins to run on the desktop.
A skin can be many things. Some skins are very simple, single-purpose tools, like Windows desktop gadgets, or "widgets" on an Android device. Others are more complex, like miniature applications themselves. Some skins even come bundled in large "suites" and include their own tools for customizing their form and appearance, within or alongside Rainmeter's basic user interface. Every skin works differently, depending on the choices of that skin's individual author.
However, all skins are made from the same building blocks: measures, which gather information from your computer, a website, a text file, or some other source; and meters, which create visual elements in the skin's window, such as frames, borders, backgrounds, images, text, charts, or buttons.
Skins can interact with other skins and applications using special commands, called bangs, and they can be customized by changing short lines of text, called variables. All of these things are made possible by Rainmeter's unique code language, which allows a skin to access functions and resources built into the Rainmeter application. Every skin's code is completely open, and can be tweaked, modified or even completely rewritten using any text editing software.