LevelWhat level are you on?
The world structure of a game is how levels are connected and how the player can move between them.
Linear: In a linear structure the levels are always in the same order. You are at level 1 and then level 2 and level 3 and so on.
Branching: With a branching structure there might be more then one way to go in a level. You can find a secet that takes you directly from level 1 to 3 for example.
Hub: In a hub structure there is a level, the hub, that give access to many other levels. After visiting a level one can return to the hub and select a new level. One example is Zelda games where to overworld act as a hub and the dungeons as levels.
Open World: This structure gives the player the ability to move freely in the world and approach objectives as he wish.
Composition in Level Design - 2014
The Iterative Level Design Process Used to Ship Fallout 3 and Skyrim - 2014
Keeping Your Players Engaged - Tips for Great Game Level Design - 2013
Beginning Level Design Part 2: Rules to Design By and Parting Advice - 1999