Youth system is a sporting terminology used to refer to a youth investment program within a particular team or league, which develops and nurtures young talent, with the vision of using them in the first team if they show enough promise, and to fill up squads numbers in some teams with small budgets.
In contrast to most sports in the United States where the collegiate system is responsible for developing young sports people, most football clubs take responsibility for developing their own players of the future. The English Premier League is known for having a large youth league attached to its senior clubs.
Most youth systems attached exclusively to one club are often called youth academies. In a youth academy a club will sign multiple players at a very young age and teach them vital skills required to play at that club's standard. While similar to youth academies, youth leagues are built into a club competition rather than an individual club.
The purpose of these leagues is to give young players experience in proper competition against other players that they will most likely end up playing with or against. In the forthcoming season the A-League in Australia will begin its own youth league. Youth leagues aren't just exclusive to association football though.