Of sound or style, "Indie" means sounds like Oasis (and since all Oasis sounds the same that kind of focusses the concept).

It distinguishes from (not a complete list, and I could not necessarily distinguish of define many of them)
Metal
Nu-metal
Death Metal
Speed Metal
Norse Metal
Grunge
EMO
Screamo
God Rock
Heavy Rock
Pomp Rock
Soft Rock
Pop
Britpop (although Indie is relatively close to this)
Power pop
Punk
Old School

About 99 different types of dance and trance which all sound very similar to me
About 99 other different types of Mobo which all sound very similar to me
About 99 different types of Latino which all sound very similar to me

Folk
Country
C+W
AOR
MOR

Indie usually has that characteristic annoying nasal whine of Oasis, enough distortion on (usually two) guitars to conceal (usually) limited technique, probably single drum pedal, and kick and bass not overpowering in the mix (and probably very limited FX on bass).

"Exactly" is as hard to tie down here as it is in other musical issues.