Atlassian gadgets are similar to Google gadgets. A gadget is a small object (i.e. a piece of functionality) offering dynamic content that you can put onto a dashboard, a wiki page or some other web page. Atlassian gadgets allow interaction with Atlassian applications such as JIRA and Confluence. All Atlassian gadgets support the OpenSocial specification.
A dashboard is usually the landing page of an application such as JIRA or Confluence. With the newer Atlassian dashboards, you can add gadgets and personalise the dashboard display by moving the gadgets around, changing their colour, and changing other preferences to suit you.