When I talk to people about Mashups, I find many of you are really thinking of iGoogle-type, dashboard-style applications, assembled using Gadgets and Widgets.
This is an accurate visualization in many cases. Yet it's important to remember there are really two different pieces here: One is related to back-end content integration and the ability to access content from multiple sources, while the other is related to the presentation layer and displaying content.
A Mashup is about content and data integration. While there are many ways of integrating content, Mashup products try to make it easier, especially if the external source exposes content in web-friendly ways such as RSS feeds, REST calls, or formal Web Services. Once you establish a base connection to the external source, these products also provide a designer (usually visual) to model how content will flow, along with the ability to transform, manipulate, filter, and extract stuff from these feeds. You can also join multiple feeds and manipulate them.
The result is then published as yet another Web Service or RSS feed. You can now take this feed and display it via your Portal or another application.
You can then use Gadgets to display these feeds.
Where things can get interesting is when you employ a Gadget to aggregate content from multiple sources to create a composite application. Some Gadget products provide you a client-based event framework where one Gadget can communicate with another (such as when you enter an address in one window and it shows that address in Yahoo maps in another window). But other than that there's no real mashing up happening here.
As you can see, there's some degree of overlap between the two categories of tools. Mashup tools provide support for these presentation components using some sort of Gadget-like technology and on the other hand, Gadget tools provide a way to wire together Gadgets using an event framework. While some people call this behavior "client-side Mashups," we consider Mashup services as quite distinct from Gadget capabilities. And so should you, regardless of what your vendor tells you.
We will be reviewing the major Mashup platforms in our forthcoming update to the Portals research.