To add to what others have said...
Marketing hype. They are 50% Florida and 50% Northern (native) bass. People in the fisheries industry call them F1 hybrids, meaning first generation cross of Florida/native genetics. There are "Tiger Bass" in every lake in Texas that has had Florida bass introduced to mix with the natives.
They are not expensive, and are a good option for pond stocking if you are looking for all around fishing (good numbers, decent size). If you are looking to grow the biggest bass possible, you will want to stick with pure Florida genetics, though. If you are looking for pure numbers, northern (natives) would be better. F1s fall in between the two. More aggressive than a pure Florida, but bigger max size than a native.
The name "Tiger Bass" is 100% a marketing gimmick.