Here are just a few of the more popular types of plagiarisms. For more information you can take a look at the resources,
Direct Plagiarism is when you copy another person's work, word-for-word, without citing it, or when you submit a paper that someone else wrote and passing it off as your own. You are essentially presenting that information in a way that says you came up with it, it is a deliberate attempt at plagiarism.
Accidental or Unintentional Plagiarism happens when a person misquotes, paraphrases or fails to cite their sources. Most instructors will have no idea you plagiarized unintentionally, but it will be considered plagiarism.
Self-Plagiarism (yes, you can even plagiarize yourself) happens when you use an assignment that was used for another course and you decide to use that assignment to supplement a different course with a similar assignment. If you feel that you should be able to use an assignment you did for another class, you would need to contact your instructor for permission to do so. However, if you are using a quote or paraphrasing from your assignment in another, you will need to cite yourself.
Common Knowledge is not plagiarism. You do not need to cite common knowledge. Common knowledge is something that the readers should generally know, such as who is the current President of the United States.
*General rule of thumb: It can be considered common knowledge when that information has been in at least 4-5 other works and not cited. Always verify with your professor.*