Plagiarism (lat. plagium 'kidnapping') is the illicit appropriation of another person’s intellectual property. 1
In other words, plagiarism is the stealing of ideas. 2 That means you can’t just borrow text without giving sources!
When individual words, passages of text or structures are borrowed in writing, the exact source and the original author must be correctly cited.
“By plagiarism we mean the appropriation, in whole or in part, of somebody else’s work without citing the original source and author. Plagiarism is essentially a violation of copyright. Brief passages of somebody else’s work may be cited, so long as the citation is properly labeled and the source is indicated.” 3
In general, it is not forbidden to refer to somebody else’s ideas; however, you have to abide by certain rules of fair play (see: Preventing plagiarism).