If you didn't write it, you have to cite it!
Documenting the sources used in your research is important! Whether you use a direct quote from someone else's work, or paraphrase (re-write in your own words), you must give credit to the original author.
Representing another author's words, thoughts, or ideas as your own is called plagiarism, and can have academic consequences.
Other types of plagiarism include:
The infographic at the bottom of this page provides some tips for understanding and avoiding plagiarism.
Citation Styles
Plagiarism.org ("What is citation?") defines citations this way:
There are many different types of citation styles, including APA (American Psychological Association), MLA (Modern Languages Association), and Chicago/Turabian. When you get to high school and college, most teachers/professors will require that you use APA or MLA and you will need more in-depth understanding of how to format citations. You can learn more about APA and MLA in this research guide.
It is good to know that you will never have to memorize how to format citations. Even after years of practice, you will probably have to check the manuals / style guides for correct format and use of punctuation. Style guides are updated periodically, so you'll always have to make sure that you are following current guidelines.
These free, online citation generators may also be helpful, but you should always check generated citations against the appropriate style guide, for accuracy.