re: What's the difference in "Alternative rock" and "Indie rock" music?

Indie rock doesn't really exist any more. It was a term used for "underground" bands that were either on indie labels or couldn't be heard on the radio. The sound was more lo-fi and garagey in the 90's and early 2000's. A lot has changed since then and it's hard to classify any music as "indie rock" these days.