It’s plagiarism if you use someone else’s words without acknowledgement. If you use someone else’s words, not only must you give the source, but you must also put these words in quotation marks or use some other appropriate means of indicating that the words are not your own. This requirement applies to spoken as well as written words and mathematical formulations, whether or not they have been formally published.
It’s plagiarism if you use someone else’s ideas, facts, data, or arguments without acknowledgement, even if the words you use are your own. If you use someone else’s examples, reasoning, or experimental results, you must acknowledge that use. Paraphrasing, summarizing, or rearranging someone else’s words, ideas, or results does not alter your indebtedness to the source, which must be acknowledged.
It’s plagiarism if you acknowledge someone in a way that will lead a reader or listener to think your indebtedness is less than it actually was. If you use a whole paragraph worth of ideas from a source and include as your final sentence a quotation from that source, for example, you must indicate that your indebtedness includes more than just the quotation. If you simply put a page number after the quotation, you will lead your reader to think that only the quotation comes from the source. Instead, make clear that you have used more than the quotation.
The Purdue University Online Writing Lab (OWL) offers this site explaining how students can avoid plagiarism. Provides a brief overview of contradictions in academic writing, actions that might be seen as plagiarism and guidelines for researching, quoting, paraphrasing, and deciding if something is common knowledge. Includes exercises and a brief bibliography.