Tier

The tier-system is a system used to categorise teams into groups that are of equal capability and often times compete against each other.

There is T1 - Teams at the highest level. They play at the world stage and are the most competitive and the highest earning teams.
Examples: G2, Sentinels, Fnatic

There is T2 - Teams just below T1. They play in the next smaller tournaments and are often times sister / academy teams of their T1 counterparts.
Examples: Gen.G Global Academy, G2 Arctic

There is T3 - Teams just below T2. They are notable but usually cannot compete with the majority of the T2 teams.

There is (sometimes) T4 - Basically everything that regularly competes in tournaments with T3 but is not as successful.