The U21 Champions Cup is a tournament for the winners of the U21 Youth Cups of all countries. There is no division in continents in this tournament, so every country in the world competes in this tournament.
In bigger countries, the runner up of the U21 Youth Cup also qualifies for this tournament. You can see if this is the case in your country on the page of the U21 Cup in the right column below the link to the U21 Champions Cup.
The two finalists of last years U21 champions cup also qualify, if they don't qualify in their own country.
The tournament is played in the same format as the U21 Youth Cup. Seeding is done based on the popularity of the teams. Teams with a higher popularity enter the tournament in later rounds.
Each player can get experience in his first 3 U21 Champions Cup matches each season. For each of these matches he'll get 600 experience points.