The word 'rap'
The word 'rap' dates back to as early as 13th century. The word was probably borrowed from Norway, where ‘rap’ or ‘rapp’ meant, and still means, 'light blow'. In the 15th and 16th centuries in Great Britain, the word 'rap' was defined as to strike or hit something, to punish someone or to criticise sharply.
In the early 20th century it became a term for 'talking', 'speaking' or 'chatting'. By the 1960’s, the word had been adopted by African Americans, meaning to have a conversation.
What is Rap?
In How to Rap 2: Advanced Flow and Delivery Techniques by Paul Edwards, rapper Del the Funky Homosapien describes rap music as:
I was born in '72 and back then what rapping meant basically, was you trying to convey something - you're trying to convince somebody. That’s what rapping is, it’s in the way that you talk.
Ray Charles about rap music:
The truth of the matter is, all rap is is talking to the beat.
Difference between rap and hip-hop
Rap is a vocal technique, while hip-hop is a subculture. Rap is one component of hip-hop culture, which includes the rap (the MC), the beats (the DJ), break-dancing (b-boying), and graffiti art. Afrika Bambaataa was one of the most important figures in the early development of hip-hop and articulated the four elements that made up this subculture.
And KRS One explained the difference as follows:
Rap is something you do; hip hop is something you live
Source: Today I Found Out, Wikipedia, Word.tips
Source image: Chris Toms via Flickr