I agree that a search would provide some info, but to someone not familiar with the subtleties of bicycle mechanics, it may not do much good. As to why you're asking might be helpful though.

I assume you're looking at a frame purchase and don't know which way to go? BB68 and BB30 are distinctly different. BB68 used a threaded shell (English) in which the bottom bracket is fixed via a thread, be it a cartridge type, or outboard bearings with a separate spindle. BB30 uses a larger shell with the bearings pressed directly into the shell, and a spindle of a larger diameter.

Which way to go? Doesn't really matter. The English shell will give you more of a crank selection, and have less expensive options. BB30 will have fewer crankset options (unless you go with them damned adaptors). BB30 will save some weight overall, since the cranksets are generally lighter. If you aren't able to do your own wrenching, I'd probably steer clear of the BB30/PF30 type.