Adventure Mode is a Game mode created by the Mojang team, and it was first introduced in Minecraft's 1.3.1 release.
Inside of adventure mode, one can only break blocks with the right tool, for example, a pickaxe cannot break a Log, a shovel cannot break stone, and an axe cannot break Dirt. This makes custom maps much more challenging since contestants cannot cheat. This does not prevent a player from killing mobs, however, as a player can kill mobs with any tool or item. One can set their game mode to adventure when opening a world to LAN, or in-game by using the command:
/gamemode @p 2.
Adventure mode is useful in adventure maps and custom servers; so a player cannot mine walls and get into places where they are not supposed to be.
In 1.8, adventure mode was made even harder; players couldn't break any blocks with any tool anymore, and they couldn't place them either. What players would have to do now is put a NBT tag on a certain block in order for it to be placed on anything (for example, a button with a "Can be placed on NBT tag", can only be placed on that or these blocks), so a player can't place the block on Stone if its NBT tag was dirt. For tools (Axe, Pickaxe etc.) there's a "Can Break" NBT tag, which allows a player to do anything with that block's breaking purpose (A sword with the "Can Break" tag for Netherrack will allow the sword to break Netherrack).
- Adventure mode was originally called "Dungeons and Levers".
- Adventure mode is the fourth game mode added to Minecraft. The first was Survival, the second Creative, and the third Hardcore.
- Due to the mechanics of adventure mode, it's often used to prevent cheating inaptly-named "Adventure" maps, in which the goal is to reach the end by doing a series of trials, which can include challenges such as: parkour, puzzle-solving, outrunning mobs, etc.