Lock

Locks are found in almost all dungeons that Link explores in every Zelda game. Some locks are impassable without completing certain puzzles, while others require keys to open.

Bar Lock
A Bar Lock is a kind of lock on a door usually found in all dungeons. These locks cannot be opened unless a certain task is completed. For example, defeating enemies, moving a block/pot onto a Switch, etc. They are called bar locks because of the metal bars that drop down in front of the door to prevent Link from opening it. In a mini-Boss room, the bars drop down when Link enters the room and lifts up when the enemy is defeated.

Locked Door
Locked doors, which are mostly found in dungeons, require a Key to open. The keys from a particular dungeon can be used only on doors within that same dungeon with the exception of the original Legend of Zelda in which keys could be used in any dungeons.

Key Block
Key Blocks are locked blocks that appear in Link's Awakening, Oracle of Seasons, The Minish Cap, Oracle of Ages, and Four Swords Adventures. These blocks will stop Link's progress through Dungeons and paths, and they can only be opened by using a small key.

Boss Lock
Boss Locks are special locks to the bosses in The Legend of Zelda series that requires Link to find a Boss Key, also known as a Big Key.

Originally, as first introduced in A Link to the Past, the Boss Lock opened up to yield a couple final puzzle rooms before the actual boss room. As the series progressed though, the distance between the Boss Lock and the actual Boss Room shortened, and eventually, the Boss Lock became the very final obstacle Link faced before attempting the dungeon boss. Link's Awakening marks the first appearance of true Boss Locks, of which are opened with a Nightmare Key. Every level has a Nightmare Key and a Boss Lock right before the Boss Room. Ocarina of Time, Majora's Mask, and The Wind Waker boasts Boss Locks in the temples (all dungeons in The Windwaker), and have the final puzzle rooms, made familiar by Link's Awakening, placed before the Boss Lock, and by extention, the Boss Room, as well. However, the final battle of Majora's Mask, in the Moon, does not incporporate the Boss Lock, unlike Ganon's Tower of Ocarina of Time, as Link must talk to the child wearing Majora's Mask to proceed directly to the final battle.

With the introduction of Twilight Princess, one Boss Lock lost its static appearance in favor of setting and atmosphere. The locks are all very similar except for the bedroom lock found in Snowpeak Ruins, which resembles a heart instead of the common eye-like shape.