There are a number of different devices and sizes on which you need your website to display correctly.
The first size used in your CSS is the starting point, and at the moment the standard is to design for a monitor that is set to 1024px wide (your design 980px wide). But as you are using Media Queries you could always start at a larger size. The section on Viewpoint explains the necessity of using this standard size when creating sites without Media Queries.
The second is for a tablet (ipad etc). These tablets come in a varity of screen sizes and most information can be viewed at a portrait or landscape orientation. The original ipad is 768px portrait and 1024px landscape, the Kindle is 600px
and the Nook is slightly smaller than the Kindle. There are other sizes for the but most are within this range.
Cell phones can be anywhere from 480px (iphone, landscape) all the way down to 280px wide.
Not all devices have the sames dpi. Moble devices have a higher dpi than convensional devices. This is taken into consideration and calculated for by the receiving software.