There are, around the web, some good explanations about "what is Go?" and "how do you play it?" Here below you can find some of 'em.

What is Go? [Mindy McAdams]

What the heck is Go? [Ken Warkentyne]

The Rules of Go [Jan van der Steen]

Touchstone Games rules explanation

The Game of Go [John Tromp]

Go - A Battle in Black and White [Ken Warkentyne]

A commented game on a 9x9 board. [Ken Warkentyne]

Let's Go - a neat version of the rules [Mark Willems]

Official American Go Association rules [Fred Hansen]

Go Frequently Asked Questions FAQ