The vast majority of easily available litter is made from clay. There are, however, plenty of alternatives for the environmentally conscious. Alternative materials include citrus peels, shells, corncobs, wheat, recycled newspaper, kenaf plant fibers and wood that have been shredded, manufactured into cat-friendly pellets or ground into a fine grit. The most popular type of litter is an unscented clumping type that resembles sand. Cats seem to prefer this type underfoot probably because it is soft and easy to move around.
Most cats seem to prefer sandy litter and most people seem to prefer clumping litter.
It's best to try a few varieties and then stick to one you and your cat can agree on. Clumping litters vary in how well they clump. The better the clumping, the easier it is to remove just the soiled litter. Litter that breaks up gets mixed in with the clean litter making it difficult to keep the box clean. Odorless or Scented Scented litter is for people not for cats.