they are kinda the same, alternative is more selective music such as Arctic monkey's, foo fighters, nirvana, radio-head (uncatogorized music) while rock is somewhat softer and has similar themes and beats.
Their not, but a lot of early rock and roll artists got inspiration or based their sound from the blues and jazz like Eric Clapton, Chuck Berry, The Jimi Hendrix Experience, Led Zeppelin, Santana,...
The majority of songs in this crossover category are in 4/4 time...that is, the lay back 1, 2, beat that is prevalent in good old Beachboy, Beatle, some Simon and Garfunkle, Various one hit wonders,...