A dashboard is an interactive application designed to give the user an overview of all relevant information on a single easy to digest page. Dashboard have generally been used for business intelligence and contain reports, charts, status, etc... relevant to the company's internal operations. More recently, dashboards are being used as customized portals to enhance the user's social media experience. Users can customize their dashboard by adding/removing/re-arranging modules (or "widgets").
Widgets are generally small self-contained pieces of code that can perform any number of functions from analytics, reporting, communication, or news.
On the business intelligence side, the goal is to create a single page that contains all Key Performance Indicators (KPI) for the company (or website). They are generally referred to as "Executive Dashboards" because they give enough information for a C-Level executive a sense for how the business if performing, without overwhelming them with numbers.