Not only worlds are infinite, but so are you character's progression. This means players can theoretically level up infinitely and put infinitely many skill points into their skill trees. However, this doesn't mean that players can become infinitely powerful. The power of players, creatures, items and skills doesn't scale linearly or exponentially with the level, but it scales tangentially. So while levels can be between 1 and infinity, the corresponding power is between 1 and 100. A power of 100 can only be reached with an infinite level. In lower levels, power increments are more notable than in higher levels.
This system has two advantages: First, there's no level cap and the player never has the feeling of having reached the end of the game. Second, power cannot be infinite. For each land, the game generates creatures and dungeons evenly in all power ranges (1-100). That way the player can freely travel from land to land and can always be sure to find creatures and dungeons of his power.