According to Merriam-Webster, to plagiarize is to "steal and pass off (the ideas or words of another) as one's own: use another's production without crediting the source. It is also defined as "literary theft".
Forms of plagiarism:
1. Copying word for word sentences from a book, an article, the web or other creations without citing the source.
2. Buying a research paper that is written by someone else and submitting it as your own paper.
3. Paraphrasing - putting other people's ideas into your own words and not acknowledging the originator as the source in the paper
During the research process you should have collected and organized each source used to build your research paper. During the writing phase in-text citing is critical to crediting the source of the words, phrases, sentences, or ideas you use in the paper. The final part of the research paper includes a cited sources page. Here, you will list all the sources used.
Reasons to cite:
1. Most importantly you are crediting the source (the author or creator of the work)
2. Other readers/researchers can look up the sources used
3. You are providing evidence or support of your own argument
4. Professionally, it is a way to link your research with other people's research
APA Style is used in the following fields: Social Sciences, Psychology, Sociology, Nursing, Criminology, Business and Economics
MLA Style is used in the following fields: Literature, Language, Cultural Studies,Literary Criticism, and English Studies
Publication Manual of the American Psychological Association by 
A Pocket Style Manual by