This is one of the most blatant and easy-to-spot styles of plagiarism. Essentially, a writer takes an entire written work or large chunks of text from that work and places it into his own paper. He then claims the work as his own. It is not hard to spot copy-and-paste plagiarism at work, as a quick search engine inquiry will often show exact lines of text from a copied work. Additionally, educators can usually spot tone differences between new work and what a student has previously turned in.
While a great deal of research involves reading information and putting it through your own creative process, there is a thin line between establishing your own ideas based on your research and simply paraphrasing another author's work. This form of plagiarism usually involves re-writing a paper or paragraph so that it contains many of the same points and conclusions but appears slightly different from the original version.
There is a wide variety of services available to students on the web that will write a research paper or other essay for a small fee. While these essays may be completely original works and not plagiaristic in their own right, turning one in and claiming it as your own is still an act of plagiarism. This also applies to borrowing essays from another student that has taken a similar class and retooling it to suit your purposes. Any time you use someone else's work without citation, you are plagiarizing.
Many areas of study have been explored thousands of times by students, writers, researchers and educators. It can be difficult to find your own voice in the crowd, and you may accidentally end up with something that sounds alarmingly similar to another author's work. If you work on finding your own voice, your work should be individual even if it echoes some established points. Some inexperienced writers walk the line a little too closely and could run into issues with school administration. If you are struggling to find something new to say or feel you are close to accidental plagiarism, talk to your instructor for advice.