Tournament Brackets come in Seeded or Blind Draw Tournament formats.
Blind Draw Tournaments are usually run when players are not ranked. Examples are tavern or bar tournaments such as Cornhole, Darts, Shuffleboard or Billiards where you don't know the skill level of the players. Match-ups are based solely on the luck of the draw.
Seeded Tournaments are set up so that the results of a season or league, and even experience or skill level determines which teams would play against each other in the tournament. The number one ranked team players the lowest ranked seed, the number two ranked team plays the second lowest ranked seed and so on.