Bunions are an enlargement of the big toe joint. Under too much pressure, this joint can swell, causing the bone to stick out on the side of the foot.
Bunions are usually caused by wearing shoes (running or non-running) that are too tight. Runners who overpronate (their feet roll inward) are more at risk for bunions because they put a lot of stress on their big toe when pushing-off.
Surgery is the only way to get rid of a bunion completely, but you can ease the discomfort without going under the knife. Make sure your shoes are not too tight. Next, try to take pressure off the bunion with bunion pads (sold at most drugstores). Finally, an arch support (sold at most drug stores and running stores) may also help relieve some of the pressure. If you're still feeling pain and discomfort, schedule an appointment with a podiatrist.
Make sure you're not wearing shoes that are not long enough or too narrow. You should have plenty of room in the toebox. If you can't wiggle your toes, your shoes are too tight.