That's not a stupid question at all. I'm sure most hairdressers don't know the difference between these two. So I'll try to give an explanation:
A bleach bath uses oxydation to - umm, well - bleach out the colour pigments of your hair. Like some stains are bleached on clothes, paper is bleached, whatever.
In theory it's possible to bleach hair until it's completely white. Practically, though... umm... very, very rarely.
Bleach doesn't differ between (most) artificial and natural pigments, they're all destroyed. First the black ones (Eumelanin), then the red ones (Pheomelanin).
Due to ingredients like ammonium hydroxide and hydrogen peroxide it's usually quite damaging. Bleach breaks the disulfide bridges in your hair. A bleach bath, which thins the bleach a bit, is considered less dangerous.
Some more info on the bleach mechanism:
http://www.hair-dressing.com/hair-coloring/hair-bleaching.shtmlColour B4 has a different approach. Or, let's say it like this - it does the exact opposite: Instead of oxidation it uses reduction and is said to work only on artificial pigments - those which were added to your hair by (permanent) dyes. It 'breaks' the molecules and thus enables you to wash them out of the hair. That's why you have to rinse it out for a horribly long time. And that's also why some people who bleach their hair immediately after ColourB4 (and probably didn't rinse it long enough) end up with a bad surprise, as the bleach re-oxidized the broken colour pigments and the colour they wanted to get rid of is back... The producer, by the way, only recommends the use of a semi permanent dye afterwards to get the desired shade.
Colour B4 is said to be much less damaging than bleach. However, Colour B4 most likely won't bring back the natural colour, as the hair is usually slightly bleached from the used permanent dye. Most people get some ginger colour after the use. It is not possible to lighten hair to blonde or white with this product (except for those with naturally blonde or white hair).
(Colour B4, by the way, can't do much if you dyed your hair with a silicone dye like Live XXL and then used a curling tong or straightener.
Here's Scott Cornwall's statement.)