Is Torchlight a Multiplayer game?

Solution 1:

It's not a multiplayer game. They focused only on a single player for now.

They are currently working on a MMO which will be based on the same engine.

More details on the Wikipedia page:

Max Schaefer, Runic Games CEO and former Mythos executive producer, explained the reasoning behind releasing both a single player game and an MMO. The single player game is intended to introduce the Torchlight game world to the public ahead of the MMO. Further, it would allow the team to get a released game under their belts sooner than if they had immediately started on the MMO. Also, while the Torchlight MMO will be a separate game, it will share the same action RPG gameplay and some of the art assets developed for the single player game.

The MMO will feature some different game mechanics and a wider variety of environments, including a shared "over-world." Runic has entered into a partnership with Chinese online game developer and operator Perfect World Co., Ltd. to publish the MMO worldwide.

Because the MMO is planned to be free-to-play, it can be seen as a "free MMO upgrade" to the single-player Torchlight game.

For the persons interested, even more details in an interview given to Gamasutra.

Solution 2:

Torchlight is not a multiplayer game, however Torchlight 2 will feature multiplayer and will be released Spring 2011. It will also feature full mod support in multiplayer as well.

From Torchlight 2 Website

CO-OP ACTION The most exciting new feature of Torchlight II is the multiplayer element. Play with your friends, and use the free peer-to-peer matchmaking service to meet new players of a similar level and interest. Together players will discover expansive randomized outdoor areas and deep ominous caverns; random instanced and persistent dungeons, a huge variety of all new monsters and villains, endless variations of loot to find, and quests to complete. Players journey through the perilous Overworld, enjoying all new weather features, time of day events, and a rich and cunning story to drive gameplay.

Multi-player – Play with your friends over a local area network, or over the Internet. A free matchmaking peer-to-peer service lets you make new friends for exciting co-op play.

From PC Gamer Torchlight 2 Interview with Runic Game founders:

Max Schaefer: It’s peer-to-peer co-op play with just your friends. It’s a halfway step to the MMO.

Travis Baldree: It’ll be free online play. It’s not a multiplayer extension for Torchlight, it’s a full sequel, with lots of new stuff that wasn’t in the original game, but it’ll have full multiplayer support. You can play through the singleplayer story in multiplayer co-op. We’re playing multiplayer in the office right now. We’re trying to basically look at all the things people wanted out of Torchlight, that we didn’t ship in the initial game, and we’re trying to incorporate as many of those as possible.

UPDATE
Torchlight 2 will offer free mod tools, full mod support in multiplayer

But because of the addition of multiplayer, I was concerned about how mods will be shared now. The rep confirmed to me that players will be able to use the new multiplayer framework to find games incorporating mods but, better yet, all XP earned in these games will stay with your character. I was promised persistence across all games, be they mods or standard games. As stated in the quotes, multiplayer will be free and will resemble Diablo 2 more than Hellgate: London.

Runic Games CEO, Max Schaefer, believes that Torchlight 3 will be released before Diablo 3

Runic Games CEO Max Schaefer respectfully believes that a third Torchlight game could be out before Blizzard's Diablo 3 -- a joke we'd made when the second Torchlight was recently announced. The executive tells us that the efficient size of the Runic team, which has a corporate goal to never grow above 40, would have the ability to get out another significantly upgraded sequel before Blizzard's behemoth arrived. Torchlight and its sequel have both had brisk development cycles lasting less than a year.

I would assume that Torchlight 3 will also be a multiplayer game.

Solution 3:

Torchlight is not a multiplayer game, they claim the next one will be.

However, there's some interaction between your characters, if you play more of them. There's a common stash in the town, which allows you to pass items between them. This is really cool if you play with a certain char and find a good item for another char.