What happens if I check in to a flight online, but then miss the flight?
Back in the old days, you checked in for a flight when you arrived at the airport. The airline then could see that someone was missing, and give your seat to someone else. Now, if I check in for a flight online 24 hours early, the airline doesn't know if I'm sitting at the gate waiting to get on the plane, or sitting in traffic trying to get to the airport.
My question is, if I elect to check in online before I get to the airport, and then I don't get on the plane, what happens to me? Will they let me board the next flight? Will they make me pay a change fee? Will they throw away my ticket and make me buy a new one?
If a general answer is not possible, let's say this is for a U.S. domestic flight on Delta.
Checking in but not boarding the plane is a "no show" in industry lingo. There's no Delta-wide "policy" on what happens next, this will depend entirely on the small print in your fare rules. Broadly speaking, assuming you've got a non-refundable fare:
Domestic flights: You'll get credit worth the value of your ticket, minus a change fee (typically $200). So if your fare was under $200, you lose it all. There may also be an additional condition requiring that any new ticket you purchase with the credit cost at least as much as the original.
International flights will typically contain the following boilerplate in the rules:
IF THE TICKETED FLT RESERVATION IS NOT CANCELLED PRIOR TO TICKETED FLT DEPARTURE TIME/ REBOOKING IS NOT PERMITTED AND TKT HAS NO VALUE
...which means what it says, ie. you're screwed. So either call ahead and change your flight, or hope that wailing, gnashing of teeth and rending of garments at the Delta counter helps.
Is there a difference between "checking-in and not flying" and "not checking in at all"?
Usually no, since a missed flight is a missed flight, but this may depend on the airline and the fare rules. It's easier to call the airline at the last moment to change a flight you haven't checked-in for though.