Hardcore

Hardcore mode is a game mode variant of survival, differing primarily by the lack of the ability to respawn (instead, the world is deleted) and change difficulty. The mode was introduced in Beta 1.9 Pre-release 2 and officially added in Minecraft 1.0. The idea came to Notch from a Minecraft gameplay blog on the PC Gamer site.

Features
While playing hardcore mode, the game is set to “hardcore” difficulty, which only differs from “hard” difficulty in that it cannot be changed to another difficulty while playing the world. As of Minecraft 1.3, the "Bonus Chest" and Cheats options cannot be enabled in Hardcore mode. If the player dies, the world is deleted and the player is taken to the title screen. In the world selection screen, it will say that the world is in hardcore mode in bold, red letters, as a warning.

When the player dies, the game over screen only offers a button to delete the world save instead of survival mode’s and  buttons. The world save still exists until the button is pressed, leaving the opportunity for one to keep their world by using 3rd party tools. Exiting the client and loading the world again only reloads the game at the game over screen.

Multiplayer
Hardcore Mode has been made an option for multiplayer, and its functions are similar (moreso since Single Player now runs on a multiplayer database).The hardcore multiplayer is more realistic because in real life when you die, you can't respawn anymore. Dying while in the server does not give the player a respawn button, replaced with a button prompting the player to "Leave Server". When clicked, the player gets the message "You have died. Game over man, it's game over!", as well as: Banned from server" "Reason: Death on hardcore". If you try to reconnect to the server, it says "You are banned from this server!".

It is possible for a server admin to allow a player to rejoin after death, but the admin will need to stop the server and edit server files: First, deleting the player data from (Map name)> players > (deadplayer).dat, then going into the banned-players.txt and erasing the relevant data under the index ( banned by | banned until | reason). The admin can then restart the server and the affected player can spawn again. However, the player will appear at the original spawn point with no items, armor, or levels (because their player data has been deleted). However, you can go into the .minecraft folder and find the hardcore world save. Copy it before you die, paste it somewhere safe, and you can drag it back into the .minecraft folder to start the world from there.

Tutorial
For a comprehensive guide on how to survive in Hardcore mode, read the Hardcore mode tutorial.

Trivia

 * In the world selection screen, Hardcore mode is written in red.
 * When a world in hardcore mode is created with the Beta 1.9 pre-release, then played with the Beta 1.8 version, the world is permanently converted into a standard survival world.
 * When Notch announced hardcore mode on Twitter, he jokingly posted an image of an "Ultra Hardcore" mode where the premium status of the player's user account is revoked on death.
 * It is actually possible to respawn in hardcore mode, via defeating the Ender Dragon and activating the credits, after which you will promptly respawn, without world deletion or death.
 * Many Minecraft enthusiasts played hardcore mode before the official version was released. It was simply on the honor system to delete your world if you died. This was often called an "Ironman" game.
 * Via hacking, it is possible to play a cross between hardcore and creative mode. The player can fly and place infinite blocks like in creative mode, but the world will be deleted upon death. This is because there are separate variables determining if creative, survival or hardcore is on.
 * A simple way to access creative mode while in hardcore mode is to go to the pause screen and select "Open to LAN", and click "Enable Cheats". Then use any command you wish.