Article snapshot taken from Wikipedia with creative commons attribution-sharealike license.
Give it a read and then ask your questions in the chat.
We can research this topic together.
First-phase: the 15 participants play all against all twice, but divided into two groups (one 7 and one 8) for classification, in the Group B, each team plays two more matches against any other. The first 2 of each group are classified for the finals.
Final-phase: the four clubs classified play all against all twice. The club with most points at this stage is the champion.
Tie-breaking criteria:
1 - Goal difference 2 - Raffle
With one victory, a team still gained 2 points, instead of 3.