MAC calls it a Beauty Balm, when it's actually called a Blemish Balm in Asia.
Basically, it's a type of foundation that aims to cover blemishes, offer good coverage, and blend imperceptibly onto the skin with a satiny finish. It is said to improve skin texture over time and reduce acne scars. It is regarded as a skincare and foundation at the same time. It is rather thick in texture, and contains lots of silicones (to achieve the satiny finish). Usually colors are pretty light as it's aimed for the Asian market.
It was originally a cream that was designed to regenerate the skin of patients that underwent laser treatments. Korean celebrities started using it as a foundation, and it became popular. Asian brands developed their own BB creams. There are tons of them on the Asian market, including some developed by European and American brands (La Roche Posay, Este Lauder for example).
I don't know which one is the best because there are so many variations now, and it's not very easy to get your hands on them if you're not in Asia. It all depends on what you're looking for and whether or not you're ready to pay for somethig you can't try beforehand. A friend of mine who's a fan of BB creams loves the Missha perfect cover BB cream. The names I've read pretty often are SkinFood, Etude House, Skin79, Hanskin, or Dr Jart.
You can visit this website to now to see some BB creams:
http://www.cosme-de.com/en/product/search_results.html?srchstr=BB%20Cream
Hope that helps!