This is something I've always wondered about and maybe someone here knows. A few years ago, a trend began in music where an artist (often a rapper) would take a few beats from a big hit record, incorporate it into a brand new tune and then release it as their new single, which would get radio airplay. 

This came up because I just saw that long running TV spot from the Scarborough Health Network Foundation, which features the very recognizable opening riffs from the Guess Who's classic "These Eyes," mixed during the commercial with lyrics from someone else. I'm guessing the songwriters gave permission for use in the campaign for charity, and it's definitely ear-catching to hear both. 

But what about artists who regularly do this on their own songs, and call it "sampling?" Isn't that the same thing as theft? Do they need permission from the originator to allow this or is there some sort of weird "fair use provision" at play that lets them essentially take whatever they want as long as they don't use too much of it?

I've always thought of it as stealing and I've never understood exactly why it's seen as being OK. If George Harrison can be guilty of unconsciously swiping "He's So Fine" on "My Sweet Lord," then how do they excuse this, which is far more blatant? Are the other artists compensated for their work?  What are the laws about sampling and when does it become an outright theft of someone's material?