Locked Doors cannot be opened except by a player who has the appropriate Door Key in their inventory. Even redstone will not budge them. But get the matching Door Key and you can open it to your heart's content. Each door has a Door Key that matches it's color. Red Door Key opens Red Locked Doors, Yellow Door Key opens Yellow Locked Doors, etc.
The best part is you don't have to click the door with the key, simply having it in your inventory is enough to let you open and close the door at will.
The Locked Blocks work in a very similar way, but they are like a button. Clicking on them without the right Block Key will do nothing. But if you have the matching Block Key in your inventory, clicking them will produce a redstone pulse. Once again there is a matching Block Key for each color of Locked Block.
This lets you set up even more elaborate things to hide behind your locks. Maybe make the floor open and drop the player into a deep dungeon. Maybe texture the key to be a plunger and the lock to be the box and use it to set off some TNT that blows open an old mine. Whatever you can make with redstone can be used with the Locked Blocks.
Locked Chests should be pretty easy to figure out at this point. Each color of Chest can only be opened by a player who possesses the same color Chest Key. In all other respects, they behave like normal chests, and can even be made into Large Chests.
Skeleton Keys allow you to do it all for the that same color. A White Skeleton Key will open White Locked Doors, activate White Locked Blocks, and use White Locked Chests. Obviously every other color follows suit. Ideal for Team Leaders or an emergency backup somewhere. The Master Key opens literally everything, so probably want to reserve that one for testing a map or having on you as a server admin.
Currently All Standard Locks and Keys are Creative Mode Only. After all, it wouldn't do much good to let players lock their bases on a PvP server if the opposing team can just craft a key. I am trying to come up with a balanced idea to allow it via the Config in the future however.
Standard Locks and Keys
Simple Locks and Keys
Simple Locked Doors, Simple Locked Blocks, and Simple Locked Chests are designed primarily for Adventure Maps. If you've played Legend of Zelda, you can think of these sort of like the locked doors in a dungeon and the small keys you could find to open them. The various Blocks are Creative Mode Only, but the keys can actually be crafted (See the recipe and description section for details).
A Simple Door Key will open any Simple Locked Door, but get used up in the process. In order to keep players from accidentally using up too many keys, once a Simple Locked Door is unlocked, it will remain open forever, it cannot be moved again.
Simple Locked Blocks will emit a redstone signal upon being clicked, once again using up a Simple Block Key in the process. Once activated, they remain active, emitting the signal, unless broken. Again, this prevents players from accidentally burning through keys and being unable to complete whatever the Adventure Map objective is.
Simple Locked Chests follow this behavior as well. They are locked and can only be opened if you possess a SImple Chest Key, but once the key is used and the chest unlocked, it will open for anybody at anytime. Simple Locked Double Chests do NOT use two keys, only one is needed.
This may be less than ideal for people playing Survival, but if you really wanted to use this you could always make a Command Block that removes the Simple Lock and then places another one back. Again, these were devised by request for Adventure Map makers (who probably play way too much Zelda....actually, there is no such thing as too much Zelda, I love the idea of Adventure Maps that emulate the Zelda feel).