Rock is a harder version of rock n roll and old fashion r'n'b (I'm talking Ray Charles here not any modern equivalent)
It should have a steady thumping beat.
It should be played on electric instruments.
The guitar should be prominent.
It has nothing to do with the type of singer (negating Mr Cowell's comment about Joe Cocker) and nothing to do with the band or singer as it is really a style (so taking a Stones song and making it motown is not the point)
It has gone in phases
60's rock: Built on an RnB/blues vibe: e.g. Stones, Faces, some Beatles stuff.
Prog Rock: Later 60s/early 70s stuff: Pink Floyd, Led Zep, Genesis...
Glam Rock: Early Queen, early Bowie...
Punk Rock: The Clash, Sex Pistols...
Pomp Rock: Kiss, Bon Jovi, Whitesnake
Soft Rock: Heart, Foreigner...
Grunge: Nirvana, Pearl Jam...
Modern: Green Day, Nickleback...
There are some singers who really define the 'rock' style of singing. These people all have really great voices with a fabulous range and can sing pretty much anything well: Paul Rodgers is the king!
There are some bands that I consider definitive: timeless tracks that people of all ages know: Free, AC/DC, Thin Lizzy, Bowie, Nirvana, Queen...