"What's the difference between portabello, crimini and baby bella mushrooms, and what should I look for when I buy them?"
Botanically, there is not a difference between portabello and crimini mushrooms‚ and baby bella is just another name for criminis. But in the marketplace, you'll see a difference. Mushrooms sold as portabellos are larger, and the gills are exposed and dark, because the mushroom is allowed to ripen after being picked. Criminis are smaller, and their gills are covered with a "veil" of mushroom skin. In fact, if the gills are not covered, the crimini is not as fresh as it should be.
Both criminis and portabellos are more intensely flavored than their more widely used cousin, the white button mushroom.
See 7 Ways with Mushroomsfor recipes using different types of fresh mushrooms.