The Oxford English Dictionary describes plagiarism as "the taking and using as one's own... the thoughts, writings, or inventions of another". This means taking work from a book, the internet or someone else in your class and passing it off as your own work. Plagiarism can be very easy and tempting, if you find an article on the internet and you copy and paste a paragraph into your work that is plagarism, even if you change some of the words to your own that is still plagiarism.
You can easily avoid plagiarism, if you use information from a book or the internet surround the information in quotation marks and in brackets state the source. For example this is a piece of information on Dinosaurs; "The dinosaurs were among the most successful animals ever to live on the Earth. Their reign lasted over 100 million years - and if birds evolved from the dinosaurs, then their descendents are still alive today." (BBC website, Science and Nature, Prehistoric Life). By doing this you can avoid plagiarism and it's consequences (ask your teacher exactly how they want quotes set out). For more information on quoting see the link 'to quote or not to quote?' on the right.
If you are caught plagiarising in your school work you will probably be warned by your teacher and given a sanction. However, if you are found to have plagiarised coursework you will be treated much more harshly. The exam board can give a mark of zero for your coursework even if only a small portion is plagiarised and in some cases you may fail the whole course. It is easy for exam boards to identify plagiarised work as they have software that can compare your work against the internet and work of other students, see the link on the right for more information. Make sure your work is original and state where your information is from to avoid plagiarism, don't risk it.