I would say sculpture is sculpture, no matter the medium, and there are two basic ways (or, of course, a combination of the two) to achieve the sculpted form: taking away from, and adding to.
Sculpture by taking away from, or carving, can be boiled down to a joke (or artistic legend) that I've heard once or a million times. To carve a sculpture of a horse (or whatever), take your stone (or wood, or polymer clay chunk, or soap, or concrete, or candle wax), and carve away all the parts that do not look like a horse. ;p Wish it were all just that simple.
Sculpture by adding to is building that horse with lots of smaller pieces. Again, because the coolness of this medium is the color IN the clay, sculpture by adding to is most popular.
I haven't actually seen a lot of carving of poly clay, not at least in a traditional 3D sculpture, but more in carving stamps, and
other flatter pieces.
Using the color in the clay, building up a sculpture can be extremely beautiful. Artists can use different colors for every different detail, however tiny, or use fleshy colors to make gorgeous figures.
One of my greatest inspirations in this category, happens to also be my first cousin. Alicia has made some of the most fantastic dolls, several in the theme of her favorite musical, Phantom of the Opera. I also really love her almost unsettlingly beautiful Bird Doll, which was, I believe the predecessor to her line of Parrot Jewelry.
After all that inspiration, if you need any more of a push to get your fingers into the clay, here is a great Puppy Tutorial and a Mummy Video Tutorial.