what is patching?

i have nba 2k9 and was wondering what patching does and how i get them? If i understand correct it downloads the rosters set by who ever made the patches but does it replace the current rosters for 2009? Because i want to create an all dynasty team. i saw a patch for 2002-2003 rosters and that would be cool i need some players from that year mixed with 2008-2009 rosters is it possible by downloading 2002-2003 patch or does it just limit me to 2002-2003 rosters? can i mix and and various years and players with patchers or not? please need help