The International will separate the 18 teams into two groups. Each team in Group A and Group B will play the other teams in their groups twice, and each win will earn them a seeding point for the main event.
The two teams with the least points will be eliminated when the group stage ends. The remaining 16 teams will be placed into the Main Event tournament bracket based on their seeding points.
The eight teams with the best records will end up in the Upper Bracket, where all matches are best-of-three. The other eight teams with worse records will be placed in the Lower Bracket, where they face elimination.
Teams in the first round of the lower bracket will play a single elimination match, while matches in the later rounds will be best of 3. Teams that lose in the Upper Bracket will fall to the Lower Bracket, where they will face elimination.
In the end, the final team in the Lower Bracket will play the final team in the Upper Bracket in a best-of-five match for the $15 million first-place prize.
You can see all of the rules for match play on the official "Dota 2" website.