Knitting Gauge

"So if you know the gauge used in your pattern and the gauge listed on your yarn are the same, why can't you just knit your project with confidence?

The answer goes back to the idea of the "average knitter." In reality, there is no average knitter. Everyone knits a little differently; some people knit loosely, while some knit very tightly (it shouldn't surprise you that gauge is also referred to as tension). When you give the same yarn and the same sized needles to two different knitters, the odds are very good that they will come up with a different gauge, and they might both be different from the gauge listed on the yarn label.

It's important to check your gauge before you start a project and see how your knitting compares to the gauge of your pattern."

From  Knitting Gauge: Why Knitting a Gauge Swatch Matters

By Sarah E. White, About.com.