Pillow-top mattresses have an additional layer of foam or padding atop the regular mattress. Sometimes they will be double-sided and have this layer on both sides, in which case the user is expected to flip the mattress over regularly for maximum comfort and lifespan of the mattress.
The general consensus is that pillow-top mattresses at their best are far more comfortable than standard mattresses, very good for the back and warm in Winter. Some common problems arising from bad manufacturing can be sinking accompanied by hill-like lumping and overheating in Summer particularly with latex pillow-tops. Another common complaint is that fitted sheets tall enough for the mattress are generally somewhat more expensive and not any better quality than standard fitted sheet sizes.