A widget is a chunk of code that can be embedded into any web page.  Of course you have to be the owner of the web page and cannot add widgets to any other sites.  What the widget does is feed fresh content, images, videos, etc. to your site.  Widgets are not static and will always provide dynamic, fresh content for your site from a 3rd party.  Widgetbox.com has a ton of real estate related widgets on their site today: http://www.widgetbox.com/tag/real+estate 

You can add widgets from sites like YouTube for your home tours, flikr for static home pictures, add mortgage calculators, add breaking news around real estate, etc.  There is no maintenance for widgets once they are loaded to your site.

I'll walk you through an example of how easy it is to add a widget to your site or blog.  We'll use vflyer for the example. 
1. Click http://www.widgetbox.com/widget/vflyer
2. Begin to customize your widget with the name, dimensions, colors.
3. Click "Get Widget"
4. If you would like to add the widget to your site, click "Get Widget Code".  Simply copy the code form either snippet (I'd recommend "Flash embed code".  If you would like to add the widget to something other your site, say a blog or social network, select the option you want and follow the steps.
5. Open up your site and locate where you would like to place the code you copied from the previous step.
6. Simply paste the code into your site and publish.

Visit http://www.livepads.wordpress.com for more handy tips to enhance your real estate site.