Two varieties of avocados are most commonly available in the United States. The Hass avocado, grown mostly in California and available year-round, is oval shaped with pebbly textured green skin that turns black upon ripening. The Fuerte avocado is pear shaped, larger and has thin, smooth skin that remains green as it ripens. It's in season late fall to spring and is grown primarily in Florida.
Look for unblemished avocados that are heavy for their size. Judge ripeness by pressing the avocado between your palms; it will yield to gentle pressure if it's ripe. If it's not ripe, let it stand at room temperature for a few days. Or speed the process up by placing the avocado in a paper bag and letting it stand on the counter; check it daily for ripeness. Refrigerate ripe avocados if you won't use them right away.