Raab is the loosely used term for the flowering stalks of kale, collards, turnips and Brussels sprouts that appear in winter and early spring bunches as the plants go to seed. Broccoli raab or cima di rape is a variety of broccoli that has been breed to have long, tender stalks, small florets and tender leaves. It is classically used in a dish with orrechiette pasta in the province of Puglia in Italy. Spring Raab is a variety of broccoli raab commonly seen at markets and CSAs.
Raab tend to be quite sweet in late winter and early spring though as the temperatures climb they get stronger and have a bit more spice and bitterness. Later in the season you want to be sure to blanch them before sautéing to mellow just a bit.
Whatever you call these categories of greens, they are delicious, and quick-cooking with a short season when little else is available on the greens front. The easiest preparation is to sauté the blanched greens in a little olive oil with some garlic and chili flakes or a smashed anchovy or two. They are also delicious roasted or as a pizza topping or as part of a frittata or other egg dishes.