"A full featured and beautiful CSS (Cascading Style Sheets) framework which combined the best of Blueprint, Tripoli (hence the name), Hartija's print stylesheet, 960.gs's simplicity, and Elements' icons. BlueTrip gives you a sensible set of styles and a common way to build a website so that you can skip past the grunt work and get right to designing."
A grid system is a set of css class and id that you can use to quickly create complicated layouts. When using a grid system, you can define the aspect of the page by just dropping the appropriate class in your markup. BlueTrip creates an invisible grid within your page, and provides you with a 24-column grid inside of a 950px wide container. You can use this grid to set the width of element by just changing their class.
The advantages of the grid system are many :
Flexibility: Want a 4-column layout? Or perhaps a 3-column with an extra column of white space inbetween each? You got it.
Browser Compatibility: Fully tested.
Speed: Do anything you can do from scratch in half the time.
If you're still confused, check out the source on the grid examples here. You can also see a demonstration here.
BlueTrip Feature List
Sensible typography styles
Clean form styles
A print stylesheet
An empty starter stylesheet
Status message styles
Hover infos on span for admin (from 960)
Show/Hide link for admin (built for Drupal)
Build from the BASIC 2 theme for Drupal, and therefor include all Basic 2 features, like body classes, clean id/class system, etc...