Difficulty: There are three types of difficulty:

Peaceful: Health will regenerate and hunger will not go down. Useful if you are on creative or if you are a beginner.
Easy, Normal and Hard: Health will regenerate upon full hunger or at least 9 hunger bars are full. The higher the difficulty the stronger to mob will attack.

Minecraft is a block-based game that has a variety of game modes. Some of it are: 

Survival: Survival is a game mode where your main objective is to survive. In this game mode your hunger will decrease according to your difficulty.

Creative: Creative is a game mode where you can do anything you want. You will get unlimited blocks and blocks that are only creative obtainable.

Adventure: Adventure is a game mode where you cannot break blocks unless with its appropriate tool. This game mode is used most commonly in PVP or in adventure maps.

Hardcore: Hardcore is a game mode where you only have one life and your difficulty is locked on Hard mode.