They can sign players this window, but are then banned from signing any players for the next two windows, unless they manage to get it delayed by an additional window (as Barca did to sign Suarez), something to do with the dodgy deals they've done to sign academy players.
As Barca showed....a one window ban means very little, something like 898 players were registered by them this window! Edit - still a step in the right direction though, would just like to see some more draconian measures for mega piss taking
It's technically not a transfer ban, it's a registration ban. So they can still sign players, but they can't register them to the club until the bans finishes. Which renders the ban ****ing pointless, really.
According to the Sun Dream Team news this morning, several premier league clubs could be facing the same.