Simply put, pu-erh is a class of fermented teas primarily grown and processed in Yunnan Province. There is a lot going on with pu-erhs which I can only but summarize here, though perhaps unsurprisingly, the wikipedia article about pu-erh (redirects to pu'erh tea) contains a lot of detailed and accurate information.
Two basic types of pu-erh are called sheng ("raw"), and shou ("ripe"). Sheng is generally made for aging, and is often stored for several years before being sold. Shou, on the other hand, is quickly aged in a process where the heat and humidity of the storage location is controlled, and the leaves are subject to something called "wet piling", where the piled leaves are turned to ensure even fermentation in a manner similar to composting.
For reasons relating entirely to personal taste, I've been collecting almost exclusively sheng pu-erhs. For now, I am mainly selling portions of cakes I've bought, though I probably will be buying more whole cakes in the future in sufficient quantity to re-sell. I've also accumulated pu-erh in other forms, such as mini cakes and bricks, which are sold whole.