Radishes are a part of the mustard family. Likely, the radish originated from Asia or the Mediterranean and has long been appreciated worldwide (even since antiquity!). A root crop, radishes will shoulder like onions when they are ready for harvest.
Radishes can quickly overmature if left in the ground too long. To check if they are ready, dust some soil away and check to see if a small bulb has formed, poking out from the dirt.
Raw, cooked, or pickled, radish is great to add a bit of spicy freshness to any dish!