Taken from the BBC, here is a summary of the transfer and loan rules:
Premier League and SPL clubs now have until midnight on Monday, 1 September to sign players after the deadline was extended by 24 hours for the convenience of clubs as the previous date fell on a Sunday.
English Football League and Blue Square Premier (Conference National) clubs have the same deadline for full transfers, but not for loans. Clubs in the English non-league pyramid below the Blue Square Premier are not bound by the transfer window system.
After midnight on 1 September, Premier League and SPL clubs cannot sign any players, on loan or full transfers, until the next transfer window in the 2008/09 season opens on 1 January. This window will stay open until midnight on Monday, 1 February.
Clubs in the Football League can sign players on loan outside of the transfer windows, following a seven-day period after the transfer window closes.
Outside of the transfer windows, any club may still sign players on an emergency basis (such as if they had no goalkeepers available) with the permission of their competition's governing body, ie the Premier League.
Players who are out of contract and do not have a club, can be signed outside of the transfer window if they are unattached free agents when the window closes.