Pu-Erh is like the champagne of tea in many ways, but mostly in that it's commonly expensive, it varies surprisingly, few really truly understand the processing involved, and the name is only to be used in specific cases. In order to be a Pu-Erh, it must be fermented either through shu or sheng means in the pu-erh region of Yunnan, China by trained masters. Otherwise, it's considered "dark" tea.
Somehow, oddly, Shu and Sheng are both lumped into this category, despite being made in radically different ways and tasting nothing alike. They're both fermented, though, so they're all "Dark Tea" or "Pu-Erh," right? Wrong.
Most Shu Pu-Erhs/Dark Teas are known for their incredibly dark, rich, earthy flavors, so it's understandable when people get confused with a sheng. This is because it's, to recklessly simplify things, the difference between a fermented 'green' tea and a fermented 'black' tea.
Sheng Pu-Erh is prepared by picking whole leaves, breaking them traditionally, and frying them just like any other green tea (note: there are many ways to stop the oxidation process, as you can read about in the Green Tea page, but for sheng, it's almost always frying). After all this, the leaves are heaped up and left to ferment without drying. Now most of the time, this involves "traditionally" fermenting them, but it's absolutely not unheard of to give the leaves a starter, not unlike a sourdough. After any number of years, it's ready for drinking.
Sheng Pu-Erh is known for its light vegetal flavors and earthy overtones. Like a strong green tea with the astringency mostly sapped out, it's still preferred with a lower temperature of water, as boiling water can scorch the leaves.