It's a little bit more complicated than that. The idea of "quest" in D3 is slightly different than most RPGs. It more accurately corresponds to "level" in a game like Super Mario Bros. That is, a quest is not a property of your character, but a property of the game you join.
So there is no real difference between single player and multiplayer. All you have is a character. You start a game at a specific quest and add your character to the game. By default the game starts (more or less) at the last quest that you were on. There's a button to change which quest you are on so you can go back and replay old quests.
People on your friends list can just drop into your game, or you can just jump into theirs. All characters inherit the current quest from the game. You can turn off allowing people to join your game.
Essentially, there is no real difference between a single player and multiplayer game of D3. A single player game is the multiplayer game with the option for other people to join turned off.
But yeah, it's essentially drop in and drop out. You can take your character and join a friend who is ahead of you, or join a friend who is behind you. Or you can play by yourself at any quest that you've unlocked. Or you can take your current game and make it public, allowing strangers to join.
So there is no real difference between single player and multiplayer. All you have is a character. You start a game at a specific quest and add your character to the game. By default the game starts (more or less) at the last quest that you were on. There's a button to change which quest you are on so you can go back and replay old quests.
People on your friends list can just drop into your game, or you can just jump into theirs. All characters inherit the current quest from the game. You can turn off allowing people to join your game.
Essentially, there is no real difference between a single player and multiplayer game of D3. A single player game is the multiplayer game with the option for other people to join turned off.
But yeah, it's essentially drop in and drop out. You can take your character and join a friend who is ahead of you, or join a friend who is behind you. Or you can play by yourself at any quest that you've unlocked. Or you can take your current game and make it public, allowing strangers to join.