A basic knitting pattern where the first row in knit and the second row is purled. These two rows are repeated to make the pattern. The "front" side of the fabric will look like a series of Vs, while the "back" looks like a bunch of bumpy ridges.
An alternative is reverse stockinette stitch, where the first row is purled and the second is knitted. When working in this way, the purl side is meant to be the front.
When working in the round, Stockinette Stitch is formed by knitting every stitch of every round.
Stockinette is probably the most popular knitting stitch out there and is used in a multitude of patterns, such as:
One of the perils of Stockinette stitch is that it tends to curl when left to its own devices. Add a border of ribbing, Garter Stitch or another non-curling pattern to the edges (as in the washcloth linked above) or use the curl as a design element.