Rock music  is a genre of popular music that originated as "rock and roll" in the USA
Oh I know, I love rock music for the most part.
In the early 1950s, and developed into a range of different styles 
That's a few decades older than I thought.
Yes buddy, it drew heavily on the African-American genres of blues and rhythm and blues, and from country music.
That makes sense, they are really similar in theme and sound in some ways.
rock is song-based music usually with a 4/4 time signature 
I'm not too familiar with time signature, but cool.
Rock  was influenced by the countercultural psychedelic and hippie scene
Oh really? That's quite interesting.