The ribs are divided into the following three categories:
The first seven pairs of rib bones are called "true ribs." They connect to the spine (by ligaments) at the back, and connect to the sternum by costal cartilage in the front. Costal cartilage is elastic and allows the ribcage to expand during respiration.
The next three pairs of rib bones (8, 9 and 10) are called "false ribs." Like the true ribs, false ribs are connected to the spine at the back. The primary difference in true ribs and false ribs comes in where the false ribs connect at the front. Instead of connecting to the sternum, false ribs actually connect to the lowest true ribs.
The last two pairs of ribs (11 and 12) are the smallest of all of the rib bones, and are called "floating ribs." They get the name "floating rib" because they are connected to the spine at the back, but are not connected to anything at the front, thus appearing to "float."