Here are some indisputable plagiarism examples:
Buying a paper written by someone else and submitting it as your own;
Copying all or part of someone else's work, claiming that it is your own work;
Paying others to write all or part of the paper for you;
Claiming a group assignment as your own work, though it's a work jointly done with your teammates.
However, most of the time people fall into the risk of plagiarism unintentionally because of lack of knowledge about plagiarism. Here are some examples:
Using materials from an online resource, no matter it's a website or a social media account, without giving the Internet address of it;
Direct quoting words or ideas from books or articles without putting the quotation in quotation marks and without providing the source;
Paraphrasing or rewriting words or ideas from books or articles without providing the source.
Common knowledge is something that is known by most people (Definition from Oxford Dictionary). There's no need to cite these information.
For examples: "The sun rises in the east", "Doing exercise is good for health".
However, if in doubt, CITE.