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They tend to be interactive, focused on pulling in dynamic content rather than on presentation, and are designed to complement the content of the site.
A calendar gadget is a good example of this distinction.
Therefore, Google recommends you build all Sites gadgets using the current gadgets.* API.
See this post for an explanation: is a simple but popular Include gadget that does little more than provide an iframe for passing through other web content: See Getting Started: gadgets.* API for a complete description of gadget tags and expected contents.
The Gadget Developer Guide provides all necessary details for building your own gadgets.
Some gadgets are no more than that, iframes that pass through information from another web site.