Pu-erh (pronounced poo-air) is a uniquely fermented tea, made using an old, broad-leafed variety of the tea plant (Camellia sinensis), withered and then pan-fried to remove excess moisture like the processing of green tea.  Unlike green tea, however, the heat processing part is shortened so oxidation can occur to the tea leaves.  The tea is allowed to ferment using methods meant to mimic the way it originally was discovered while tea leaves were traveling on the backs of horse or yaks on the Silk Road from Yunnan to the Tibetan Plateau for trade.  The flavor, caffeine, nutrients, & probiotic characteristics of this unique tea made it an indispensible beverage for many in China and in many indigenous communities throughout the Upper Mekong River Region of China, Laos, Vietnam, Myanmar, and India.
Modern-day pu-erh tea is either fermented packed in clay jars, baskets, or buried in the floor of caves and allowed to oxidize and age, some for as long as 60 years. Like fine wine, certain pu-erh are considered more valuable and the flavor more desirable than others. Another method used to make pu-erh is to heap-ferment the loose tea leaves for hours to days to allow interaction with fungi, yeast, and bacteria that ferment the tea.  Some pu-erh is also intentionally inoculated with desirable microorganisms.
Pu-erh tea has many reported health benefits, including acting as an antioxidant, helping to stimulate metabolic processes (thereby increasing calories used), helping with fat digestion (therefore beneficial taken with a fatty meal), increasing mental clarity & energy, improving lipid profiles, & reducing cholesterol levels.  It is a source of polyphenols, like other teas, which are phytochemicals that can protect the body from free radical damage and degenerative processes & diseases.  Other compounds include: caffeine-producing methylxanthines (theobromine & theophylline), amino acids, & amino acid-derivatives including theanine, proanthocyanadins, gallic acid, coumaric & caffeic acids.  Theanine has been shown to help reduce mental and physical stress and improve mental function.
The fat metabolism, and general metabolism-boosting properties, have been the primary focuses of its use in the West, as well as its cholesterol-lowering effect, which has largely been explained by the discovery that pu-erh tea contains natural statins produced by the probiotic activity.  Polyphenols in the tea leaves are oxidized to create fermentation-derived compounds known as statins (a group of hydroxymethylglutaryl-coenzyme A reductase inhibitors), which have been shown to reduce low-density lipoprotein cholesterol levels & prevent cardiovascular disease.
Generally speaking, you’d need to brew this tea at about 1 tsp/cup hot water for 5 minutes and drink 3 cups/day to achieve some of these desired effects.
There is a lot of interesting information out there on the internet about tea in general, and some on pu-erh.  I learned a lot about pu-erh and its ethnobotanical origins and use from an article in Herbal Gram, published by the American Botanical Council, entitled “Pu-erh Tea and the Southwest Silk Road” by Selena Ahmed, PhD and Michael Freeman.  There are also some research studies that are summarized on Pub Med, a database part of the National Institutes of Health (http://www.ncbi.nlm.nih.gov/pubmed/).