from what i understand its a bit like what pro did years ago in asia with buying trading cards, and unleashing a formation and playing others online with it. Only fifa are doing it without buying the cards physically but instead online pokemon card trading so to speak and then playing others online with the squad you have built up and traded players for, but its more technical as they've included team chemistry system so for example a defence of the same nationality will boost the chemistry rating there for increasing your chances of winning matches.
for those who have FIFA '09 on PS3, did you notice that there are no night games, and there isnt any games played on rain-soaked fields like there were in FIFA 08 ??