Many adventure games contain more than just riddles and logic and assembling pieces of a puzzle. They might also include arcade sequences, like shooting down invading aliens, and synchronous combative duels, like a game of chess. A game challenge is any such obstacle that the player must overcome to complete the game. Much of the creativity of an adventure game consists of inventing new types of challenges. Your character is usually not the only active being in the world.
Actors are other characters that are controlled by the computer.
In adventure games, actors tend to have a very limited scope of activity. They are usually there either to provide some information or object, or as a challenge. The world is a collection of discrete locations. You character moves from one location to another, exploring and mapping the world. The world is filled with objects which your character can manipulate.