I love watching people play guitar. I can't play myself, so it's fascinated to see other people with the skill to play well.
Our modern guitar was preceded by the glittern, a Renaissance guitar.
How does it differ from modern guitars?
Instead of having six strings, it was a four-course instrument.
Only four? Must have been easier to play than six! When did six strings become a thing?
That I am not sure of.
Who invented the guitar anyhow?
I do not know who gets the credit, but the four-course vihuela and the five-course baroque guitar contributed to its development. 
I wonder what types of music were played back when the guitar was first invented. Would you happen to know?
I do not know, but it is presently used in every style of music.