Typically they stand for:
SB - Small Block
SBC - Small Block Chevy
BB - Big Block (BBC would be Big block Chevy)
Again typcially the small block designation pertains to engines with standard cubic inch displacement below 400 and the big block designation is for 400 and above although there are a couple of minor exceptions such as the Chevy 402 which can be either a SB or a BB I believe depending on the year and the chevy 396 which is considered a BB.
Other manufacturers such as Mopar and Ford fall in the same ranges, again with some minor exceptions.
