Plagiarism is similar to copyright infringement, but they differ, mainly, in that plagiarism is an ethical violation and copyright infringement is a legal violation.
Academic culture has drawn a hard line against plagiarism. Institutions have worked hard to create firm guidelines and procedures for crediting and allowed use of ideas and phrases created by others. Popular culture deals with many of the same issues. Appropriation and sampling has been an issue in popular music for decades, but has reached a new peak with the rising popularity of mash-up artists.
As a mash-up artist, Girl Talk creates music by sampling the creations of other artists. According to the website of Illegal Art (Girl Talk's provocatively named record label), "With the grand intent of creating the most insane and complex pop collage album ever heard, large catalogs of both blatantly appropriated melodies and blasts of unrecognizable fragments were assembled for the ultimate Girl Talk record (clocking in at 71 minutes and 372 samples)." ("Girl Talk")
Mash-up artists are only an extreme example of musical "collages." Similar stories have been seen in pop music since before Elvis was accused of stealing from African American culture, and even in classical music. In March, 2012, composer Osvaldo Golijov was accused of plagiarizing parts of his piece, "Sidereus," despite the fact that he had obtained the other composer's permission (Midgette).
In music and some other areas of pop-culture, the boundaries between influence, allusion and plagiarism are a little bit fuzzy and frequently being challenged. In academia, however, there ARE firm rules and guidelines. They can be found in citation style guides.