1. The first one is the 50's one as you'll probably know it, though early 60's Phil Spector stuff could be thrown in too. This was when it wasn't rock but rock n' roll!
Artist Examples: Little Richard, Chuck Berry, Elvis, Buddy Holly, Jerry Lee Lewis
2. The second is the boomer era! Hippies, drugs, psychedelia, prog rock's heyday, birth of metal, glam. The mid and late 60's, the early and mid 70's mostly.
Artist Examples: The Beatles, The Rolling Stones, Led Zeppelin, Pink Floyd, Bob Dylan
3. The third is the era of punk, new wave, post-punk, synthesizers abound, and also regrettably the rise of hair metal.
Artist Examples: The Ramones, Talking Heads, U2, Van Halen , The Police
4. The fourth began as college rock in the mid-80's and eploded in the early 90's into the mainstream. Probably also including the britpop era on it's end.
Artist Examples: Nirvana, Oasis, The Pixies, The Stone Roses, Radiohead
5. And I guess fifth gen would be the 00's rock, the various "revivals", indie, etc.
Artist Examples: The Strokes, The White Stripes, Animal Collective
Who da best.