A charge of plagiarism can have severe consequences, including expulsion from a university or loss of a job, not to mention a writer's loss of credibility and professional standing. 
Plagiarism is what it's called when a person states he/she said, wrote, researched something that someone else really did.
While plagiarism is (usually) a violation of copyright, this is not always the case (for example, the actual source may be in public domain), and, when it is, it is considered separate from other kinds of copyright violation.
Intentional plagiarism is dishonest and can get a person into big trouble.
Unintentional plagiarism isn't dishonest but it will also get people into trouble especially if you can't prove it was an accident. If you are at school, college or university you need to learn when citations are needed and how to make citations, your teachers/lecturers will help with this. When you have your own work and some one else's work in the same microsoft document use different colours and possibly different fonts for you work and other work. That way you won't mix them up.