A steek is an area of fabric that is designed to be cut. This is usually to allow you to knit in the round even when there are gaps in the fabric, such as at an armhole.
A steek is an area of additional stitches that provide space between the edge of the fabric and the cut ends of the yarn. In colour work, these extra stitches usually include an edge stitch in the background colour and columns of alternating colour (either as columns or as a chequer board) to make it easier to see the steek.
The steps in making and cutting a steek are described in separate articles.