Lock

Locks are popular staples in the dungeons of The Legend of Zelda franchise, halting Link's progress through a certain room until he either completes the room's puzzle, or locates the appropriate key to unlock the door somewhere in the previous portion of the dungeon.

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.

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.

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.

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.