The purl stitch is sometimes thought of as the opposite of a knit stitch. The purl stitch is yin, the dark, shady hillside, to the knit stitch's yang, the bright, flat open space.
It might seem like a strange metaphor, but it's actually pretty accurate. Even the look of the knit and purl stitches bears this out: the purl stitch is a raised loop, while the knit stitch is a flat half-V.
When you look at something made in garter stitch you see the bumps and flat spaces clearly, even though you performed the same knit stitch over and over. That's because the "front" and "back" of a knit stitch look different -- the back of a knit stitch looks the same as a purl stitch.
They really are two parts of the same whole, just like yin and yang. The steps for purling are the same as when making a knit stitch, but the form is slightly different.
This is the method for purling in what's known as the English style, sometimes also called American or throwing, where the yarn is held in the right hand. You can also hold the yarn in the left hand; that is known as continental style.
Just like when forming the knit stitch, your first step when making a purl stitch is to open or pick up the first loop on the needle, which is again in your left hand while the empty needle is in your right.
The difference this time is that the working yarn should be in front of the needle instead of behind, and instead of going into the loop from front to back, you go from back to front.