The Small-Polyp Stony or SPS-corals have small fleshy polyps that lay down a calcareous skeleton. They are usually either branching or plated. These are different than LPS, which are also called Large Polyped Stony corals. Both SPS corals and LPS corals are reef building corals and each polyp will build a stony skeleton, while soft corals don't usually leave a stony skeleton behind when they die.
SPS corals require different water parameters than soft corals do, for their survival, and especially for their maximum growth. They prefer to have close to zero measureable Nitrates, because these Nitrates can inhibit the building of their skeleton. The calcification of their skeleton can be slowed or stopped by higher levels of Nitrates.
Small-Polyp Stony corals use zooxanthellae in the same way that LPS and other corals do, but they require more specific conditions to grow and maintain health. Due to the reduced amount of zooxanthellae in SPS corals, it is sometimes necessary to feed the corals using phytoplankton or other coral food.
It is possible to keep SPS corals in the same tank as soft corals, but in the ocean, you don't really see a lot of mixing of the two types of corals except in sort of micro environments. Most part of SPS-corals enjoy bright light and strong currents. They are often fragile and are propagated by breaking off pieces. Small-Polyp Stonies are considered to be the ultimate challenge in keeping corals. They are certainly the most difficult types of coral to keep. But by doing a few things right within your system. It's really not that difficult to keep them.