Too generic to answer this question and also you can find lot of thread, discussions & blogs are available to get the info. Also you can find some templates & samples inside the dashboard itself, play with those component and understand them better.
Best place to get familiar with the dashboard using the tutorials.
Official Product Tutorials – SAP BusinessObjects Dashboards
Installing SAP BusinessObjects Dashboards 4.0
http://help.sap.com/businessobject/product_guides/boexir4/en/xi4_dashD_user_en.pdf
SAP BusinessObjects Dashboards 4.1 – SAP Help Portal Page
SAP BusinessObjects Dashboards Resource Centre
Please go through the above links which can help you to understand how dashboard works, install & Product availability matrix.