well:

Commerical - Focuses more on Money values, having money, having bitche's and phat rides. Having your albums widespread across the globe, artists attendting award ceromies etc..

Underground - Focuses on a more deepful meaningful concept to the raps. It's a struggle in real life. Your albums only getting local radio play and not being mass produced by a big record label.

This is more definition, but when i said, " Focuses on a more deepful meaningful concept to the raps. " - dosn't mean that commerical arists DON'T do it.