"The key to avoiding plagiarism is to give credit where it is due" (Purdue Online Writing Lab)
Bottom line, document any words, ideas, or other productions that originate somewhere outside of you. (Purdue OWL)
"Generally speaking, you can regard something as common knowledge if you find the same information undocumented in at least five credible sources. Additionally, it might be common knowledge if you think the information you're presenting is something your readers will already know, or something that a person could easily find in general reference sources. But when in doubt, cite." (Purdue OWL)