Difference is both structural ( they are branched chain aminoacids) and functional, as they bypass liver and are metabolized mostly in the kidneys, deaminating into other aminos or components that will ultimately lead to glucose or energy.
That's why they are used in liver disfunction and avoided in kidney disfunction.
It's primary dietary sources are vegetal protein and casein( that why casein is considered the best anti-catabolic protein, since you will easily get glucose from BCAAs and spare muscle).