Turnless is for people who like click-fests. Go quick, or your units will go without orders that turn.
The game clock cannot be set to a specific amount of time. You get 3 or 4 choices from none (not in turnless), slowest to fastest. That will control the amount of time each player has to complete a turn in other modes. Play around with it and find the setting you like. I advise learning on the slowest setting.
Your opponents cannot log off without being eliminated. No one can join a game in progress. You have to stop the game, go back and reload from a save if you lose someone.
Hotseat is where multiple people play turn based on a single computer. It is the 'classic' form of MP. You finish your turn, hit the end button, and give up the seat to the next person in the order. People were playing Civ hotseat before Al Gore invented the internet.