Image copyright Dave Coverly, Speedbump.com, 2003.
Citing the sources of information you used is not really hard; it just takes a bit of time. There are many different citation styles, but most students need to learn to follow the conventions of only MLA or APA. Style guides are about more than just citing sources of information or ideas. Guides can help you improve the style of your writing and avoid bias, for example.
Plagiarism is using someone else's words or ideas without giving credit to the source. Many students plagiarize unintentionally, so make sure you know that plagiarism happens whenever you fail to include an in-text citation with a corresponding entry in your References or Works Cited pages. It doesn't matter if you use someone else's exact words (quote), paraphrase, or summarize: all these formats require citation. Also, citation or credit is required no matter where you find ideas or data. It doesn't matter if you're citing a peer-reviewed article, a book published by Yale University, a web site without an author, or a Tweet. All require credit. In higher education, plagiarism can lead to a failed grade or worse; in real life, it can lead to a damaged reputation or a job loss.
See our guide Academic Integrity and Avoiding Plagiarism for more detailed information.
For additional online resources, see Using Research from the Purdue OWL, especially the sections titled Quoting, Paraphrasing, and Summarizing and Plagiarism FAQ.
The Excelsior Online Writing Lab offers excellent instruction regarding various style guides, including MLA.
Purdue Online Writing Lab (OWL) offers popular instruction regarding various style guides.