A transfer window is a set period when clubs may register new players. Most European leagues have a summer window and a shorter winter one, and the exact dates are set by each national association.
What happens in a deal
Clubs agree a fee for a player under contract. The player then agrees personal terms, passes a medical exam, and the move is registered with the league before the window closes.
Loans and free agents
A loan lets a player play for another club for a fixed time, sometimes with an option to buy. Players whose contracts have ended can often join a new club without a fee, which is why contract length matters so much.
Deals pile up on the final day because a missed deadline means waiting months. Late paperwork has ended more than one transfer.