Probably the first question on your mind is why do I need this? Rainmeter is a great way to put considerably more information right in front of you at all times without having to load certain apps. By having a skin for RSS feeds, Gmail, and Facebook on your desktop, you can save yourself a lot of needless checking while working on something else.
How would you like to have the information normally available through multiple programs all on your desktop without ever needing to load those apps? Rainmeter has a very small footprint–only 24 K in the task monitor–and can display a considerable amount of information.
It's safer than a Windows Widget. For starters, Rainmeter skins are all scripted and you get the source code. You have to open up the .INI file to make edits, like changing the settings, so you can see what the skin does, and I've not yet heard of anyone hiding malware in a Rainmeter skin. Conversely, a Windows widget is a closed, compiled app, but notice that when you install one you are given a security warning about unsafe and unknown widgets.
Hero skin, available on Deviant Art
Windows widgets are also pretty much all the same size. This is more due to its Vista legacy. Vista's sidebar was fixed in width and you couldn't move the widgets away from the right side of the screen. With Windows 7, you can drag and place widgets anywhere on the screen.
Hopefully this will eventually mean widgets of larger sizes, too. Windows widgets tend to use the same font size, and it's very small. Rainmeter skins give the option of a larger font size so you don't wind up squinting at the screen.
What's really nice about Rainmeter is the author can create his or her own graphics and then display the information inside them, as opposed to just printing text on the screen like Widgets do. One of the more common and popular clocks for Rainmeter, for example, is one that clones the clock on Android smartphones. That skin comes with the images for the clock digits and background.