The biggest clue to what these are is in the name, they are pocketed springs, the coil is placed inside it's own little pocket of fabric. This allows each spring to react independently, stopping movement from disturbing anyone else you may be sharing your bed with. It also prolongs the life of the coils which in turn prolongs the life of your mattress.
Pocket springs are generally more expensive than regular open coil mattresses but this price difference is reflected in the quality and life span of the mattress.
Pocket spring mattresses range from soft to ultra firm depending on the number of springs and the other fillings within the mattress.
Generally the number of springs reflects the firmness of the mattress, 3500 being a firm mattress, and 600 being a soft mattress. Although there are other fillings which can cause a mattress to feel softer, for example when topped with memory foam a 3500 would probably feel more like a medium yet still give the spinal support that a firm mattress would offer.
Pocket spring mattresses with memory foam are amongst the most popular mattresses at the moment, as pocket springs offer the greatest life span and excellent support with the addition of the memory foam to help relieve pressure points and comfort to the mattress.