The most versatile of cooking greens, spinach is great blanched and creamed, sautéed with brown butter, or wilted and tossed with pasta. Or use it raw in a spinach salad. You'll likely find varieties with smooth leaves (flat-leaf) and crinkled leaves (Savoy). Savoy spinach tends to be darker and less fragile than flat-leaf spinach. Baby spinach is an immature stage of either type, with a delicate texture and flavor better suited for salads than for cooking.
A pound of fresh leaves will cook down to about a cup. For a side dish of cooked spinach, figure 8 oz. raw spinach per serving.
Other cooking greens, such as Swiss chard, can substitute for spinach; it will have a fuller flavor and may take a little longer to cook.
Unpackaged spinach, whether loose or bunched, is usually fresher with better flavor than leaves sealed in plastic. It's also easier to see what you're getting. Choose the perkiest-looking bunches (with no rot or yellow leaves) and untie them as soon as you get them home.
Wash spinach, even spinach that says its been washed, well in a few changes of cool water to get rid of any sandy grit. Trim away roots and any tough stems.
Fresh spinach keeps well for two or three days sealed in a plastic bag in the fridge.