Widgets, in the web world, are chunks of code that can be copy-and-pasted into HTML, Facebook, Myspace, etc.
Some allow for custmization of content, size, shape, color while others are "as-is".
What you should know before you install a widget:
1) Some widgets (Facebook's "secret crush") can be used for malicious purposes. Install and paste with care.
2) Not all widgets are school-friendly. Stick to safe content.
3) You can't control the content once the widget is live on your site. What if something appears that offends your or your visitors?
4) Widgets die. When they do, a blank space is all that your visitors will see on the site.
5) Widgets load at different speeds. This can slow down your site. Don't over-widget.
How do you find a widget? Search for "web widgets" in google.
Here are some options. I don't vouch for the quality of their content.