1 - A deep cross is a cross that comes from a position in the field that is situated before the box line - that means that the defenders will face the ball coming in front of them, making it an easier cross to defend; yet, it can be good if you have slow wingers, no wingers, or very tall players waiting for the cross in the box; it´s also a good option for central midfielders and players you don´t want to attack much;
2 - A byline cross is a cros that comes from a position in the field that is situated after the box line, and, usually, very close to the byline itself - that means that the defenders will face the ball coming away from them, in the direction of the attacker, wich makes it harder for them to defend; it´s best used if you have fast wingers, that can quick outwitt the defenders and pull the cross;
3 - To close down a player means that every time he gets the ball one of you players will immediately close down mark upon him.