It's said that "A Picture is worth a thousand words" and I say that "A Dashboard is worth a thousand queries". You can get all the definitions and pictures of a Dashboard from Wikipedia or some books. But if you want to get a real feel of how a dashboard looks like, there are two great places where one can take lessons from:
1) MS BI VPC: This one is a good example for BI developers, as it comes with all the great examples of everything that can be developed using Microsoft BI Stack of Technologies. It also includes a dashboard for an AdventureWorks cycle, that is created using Performancepoint Monitoring module. This VPC comes loaded with an evaluation version of all the softwares starting SQL Server and MS Excel to Performancepoint which has one or another feature of Business Intelligence.
2) Realtime Example: This one is for best ways to learn of how a dashboard would look like and how it would help management to take strategic decisions based on the information presented in a form of decision support system. Someone who is fresh might not be able to quickly dive into BI tools, but there's a very nice site where one can see a Dashboard in live action.
This website is called the Federal IT Dashboard. It a magnificient website and the Dashboard presentation is majestic. This website is probably owned by the US government to publish to the public, of their IT spending on different portfolios. Even if you are a seasoned BI Developer or a Novice BI aspirant, I recommend taking a look at this website to get a real feel of how Dashboard can make a difference and how an interactive dashboard can look like.