Mirin is sweet rice wine, used primarily to flavor Japanese dishes. A recent discussion on Chowhound noted that aji-mirin is the more commonly available sweetened product, easy to find in Asian groceries and some supermarkets. And while paulj thinks Kikkoman aji-mirin resembles flavored corn syrup, it's fine for many recipes. Chowhound user jjjrfoodie uses aji-mirin for everyday cooking, but wouldn't expect to find it in $50-a-plate sushi creations.
On the other hand, travelerjjm says, hon-mirin is the "real stuff," fermented rice wine that is naturally sweet without the addition of sugar or corn syrup. It's generally more expensive, and depending on your state's laws it could be harder to get than aji-mirin (hon-mirin has no added sugar or salt, and often has a higher alcohol content, so it must be sold as regular wine, MikeG says). BigSal likes Mitoku brand mirin. "Once I discovered it, I cannot use anything else," he says. "I buy it online, but it is worth it." A final tip, from Caroline1: Even if a product is labeled hon-mirin, always check the salt content on the nutrition label to be sure of exactly what you're getting!