As Johan Cruyff once said, "As long as we have the ball, we're the only ones who can score". Yet even the most technically perfect team, with the best attaching game (a.k.a Chelsea FC), still has to win the ball first.
What is pressing?
Modern defense systems are built around double-teaming the opponent who has the ball, in order to close down the space available for dribble. Defenders also try to prevent passes by tightly marking potential recivers near the ball, and they "shorten" the field by keeping the entire team in formation no more then 40 yards deep.
Many youth teams practice something that resembles pressing, but they do it in a disorganized way: In U12 & U14 soccer, attacking players (midfielders & forwards) go after the ball whenever it comes near them, while their coaches encourage them with them commands like "go after him!" and "press!". We can't call this pressing, because it's primarily an individual or group tactic, whereas pressing refers to a planned attempt by their team to achive the objectives outlined above.
While pressing is not bound to any practicular formation (e.g. 1-4-4-2) or defense system (such as zonal defense), formations without sweepers tend to be more manageable and easier to teach to youth soccer players. These formations sensitize all players to the need for a team defense tactics.
It's importante to start by teaching players that pressing is a team defense tactic. The objective of keeping opponents from dribbling or passing requires the involvement and coordination of the entire defending team, including the players farthest from the ball.