is this a multiplayer game?

[Goose]

or singleplayer?

[Tenoshii]

This is a singleplayer game.

[AvG]

It's single player that lets you visit other players towns if they upload them. You can have some small amount of interaction with others using signs and paper messages if you like. There is also an interaction with the in game newspaper with voting and an ever updating list of players who died and what killed them.
There is also a small amount of player death that can be spawned in. If the game feels like it you can come across a players corpse in rare scenarios and they sometimes have an item that killed them, usually something overweight that killed that player.