Sampling is the process of taking part of one song and slamming it into your song. It's considered fine to be done. Inspired by the fact that Leona Lewis has sampled Penguin Cafe Orchestra, and ruined one of my favourite songs, I think it's fair to call it "Theft". If I "sample" someone else's essay, suddenly I'm hauled up on charges of plagiarism.
It strikes of a creativity-impairment. "I can't think of my own melody.. I'll just take this one from another song". Usually a song so old that it's no longer really recognisable. Do you think MC Hammer wrote the riff for "U Can't Touch This"? You're wrong, it's from 1981's Super Freak, by Rick James. He had to sue to get some of the credit he deserved for basically writing the song. I was once listening to Talk by Coldplay, when my dad overheard it and went "Why are you listening to Kraftwerk?". I was pretty certain I wasn't. Turns out I was wrong, I was listening to the main riff from Computer Love. He dug out a tape to show me this. I'd have had no idea if he hadn't have told me, there was no indication in the song that they basically used Kraftwerk's song and added some other words. At least, according to Wikipedia, they received permission to use it.
In any other industry if you "sampled" a colleague's piece, it'd be rightly criticised for outright theft, or at best, unacknowledged borrowing. You wouldn't have a comedian use other comedian's jokes without ever mentioning it. You wouldn't have a journalist lift 200 words from an old column someone else wrote. You wouldn't get seemingly endless repeats of old TV shows pretending to be new content(Oh wait, sorry BBC, I forgot. Zing. Don't think I didn't see you play Yellowstone on BBC Four, then move it to BBC Two this year. If I watched it then, I don't want to watch it again. If I didn't watch it then, it's because I didn't want to watch it. Unless the BBC assumes I just watch documentaries by accident and assumes it has to put them in the place I'm most likely to trip over them).
I wouldn't have recognised Leona Lewis as having thieved Penguin Cafe Orchestra if I didn't already know and like the original song, which is slightly obscure. I'd have thought "My, Leona Lewis knows how to write a good song! That does surprise me.", but instead I'm thinking "My, Leona Lewis knows how to ruin a good song! That doesn't surprise me.". I had to listen to the song (Pity me) to find out it's just the main riff from Perpetuum Mobile (Released in 1987), repeated, over a thudding, jarring drum beat (Not in the original song because it ruins it), with some singing over the top. And occasionally an aggressively annoying bass line. Enjoy.