For many regional knife makers this is their bread and butter knife, the volume seller. As the mainstay of a knife maker's range the bannou offers great value.
Ten thousand abilities, is what it means. As with 'santoku' the idea is you're getting better than one knife in one package - an all-rounder that can manage every task you ask of it so that a full meal can be prepared from start to finish, with a range of dishes, with just one knife.
The difference between a bannou and a santoku? Nothing specific to speak of, along with the bunka they're much of a muchness - a shortish general-utility knife ideal for home meal preparation. If you have one in mind then you should include the other two in your search.
There will likely be more variation under the bannou banner. 'Bannou' is a catch-all name for an all-purpose knife rather than one recognisable style and bannou knives by different makers may be very different knives. Regardless, you should find that any bannou will be useful every time you prepare a meal; you just need to find one that suits you. Read on.