Plagiarism is considered a serious offense in the academic community. To plagiarize means to take the ideas, words, or work of another and claim them as your own. Another word for this is concept is stealing.
Most students and professionals do not intentionally try to commit plagiarism. One example of intentional plagiarism would be to purchase a paper off the internet and present the entire document as your own. Another example would be to copy and paste an entire section of a textbook or journal article into your own work and claim that you wrote the entire document yourself. These are two blatant examples of cheating, and you should know that these behaviors constitute academic and professional dishonesty.
However, while most people are not guilty of intentional plagiarism, it is unfortunately easy to fall into the category of the "accidental plagiarist." There may be no intent to cheat or steal, but the end result is still considered plagiarism. The key to avoiding plagiarism is to carefully document your resources in order to provide the proper attribution to other authors in your work.