From Hellgatewiki.com

Hardcore characters have but one life to give for the cause. If they die, they stay dead; there is no respawning or starting over again in town. Death, whether from monsters, lag, computer failure, PvP, or anything else, is forever. Choose wisely.

Rules and Requirements

Hardcore mode in Hellgate: London is inspired by and patterned after the hardcore feature in Diablo II. Just as in that game, hardcore characters can only be created once you've finished the game with a normal (softcore) character, and hardcore characters can mingle with regular characters in towns, but can never play with, trade with, or otherwise interact with softcore characters. It's not yet known if there will be any option to loot corpses upon death, as there was in Diablo II. In any event, the Hellgate: London feature of a stash that all characters on an account can access will at least help hardcore characters immediately offload any new items they find but can not personally use.

Hardcore mode. If you die that's it. You can hang around in town adn show off how high a level you got ot, but that's it.

You've got to finish the game in multiplayer once with some character to make a hardcore character. And you have to be a subscriber. But then you can make all the hardcore guys you want. Hardcore only can trade and party with other hardcore characters.

We had a similar thing in Diablo II and thee was a large section of fans who thought nothing really mattered unless you did it in HC. It's an option in HGL, if you finish the game and show you know what's going on.
--Tyler Thompson, IGN video interview, e3 2007.

Character Creation

To create a hardcore character you must have completed the game on normal difficulty with a character on your multiplayer account. Hardcore mode is only available online to elite subscribers. It will probably not be available to anyone in single player mode, though this is unconfirmed. There doesn't seem to be much point to it though, since you can simply quit or delete a single player character if they die. Furthermore, since Hellgate: London will not include any way to play single player characters in any multiplayer mode, you can't even show off your hardcoreness to your friends.

To create a hardcore character, simply click the "okay" box for hardcore upon Character Creation. It works just like selecting their class or gender. Hardcore is forever; once selected it can not be undone, and if your character ever dies they'll stay dead, and you'll lose every bit of their palladium, and everything they had in the unshared portion of their stash.

Hardcore Gameplay

Hardcore plays exactly the same as softcore, though their economies are kept entirely separate. Hardcore characters can not play or trade with non hardcore characters, although the gameplay is identical.

Q: Will Hardcore and Softcore modes be balanced differently?
A: "I don't think so. I think when you play hardcore on whichever difficulty you choose, you're claiming to be hardcore. Thus, scaling the difficulty for you would defeat the purpose of the mode."
--Ivan Sulic

Hardcore mode requires some special play, as Flagship is well aware.

You'll play very differently. You're a lot more careful about what you're doing and getting out of danger and bad situations. What skills you pick are more about survival than killing quickly.
--Tyler Thompson, IGN video interview, e3 2007.

"There's even Hardcore mode [available online], like in D2 where you get one life and that's it," he added. "Hardcore mode was very popular to a very vocal group of people who took a huge amount of pride in having a high level guy who's never died. You tend to play the game very differently."
--Bill Roper, ShackNews Preview, January 2007.

Bill is correct in that Hardcore had to be played very differently...at first. Once players had been at hardcore for a while, and accumulated experience and great equipment, it became not much different than softcore. In late 2001 the first new lvl 99 made it in just 39 hours, with several hardcore characters just a few levels behind. The scales continued to balance over the next couple of years, and after a ladder wipe in August 2004, the first new character to 99 on three of the six realms was hardcore.

Whether this will be the case in Hellgate: London remains to be seen. Diablo II turned out to be a lot easier than most of the designers intended, mostly due to item inflation (mudflation) through patches, and they've made noise about addressing that in Hellgate. Hellgate won't offer so many ways to "leech" hit points, and monsters should be able to adjust their difficulty more on the fly. The Hellgate Team also wants to cut down on experience sharing and make it harder for low level characters to party and level up by letting higher level characters do all the killing. Just how this will work out in the actual game remains to be seen, of course.