The general definition most people use is: it is the practice of taking someone else’s work or ideas and passing them off as one’s own. And according to the Merriam-Webster Online Dictionary, to ‘plagiarize’ means:
To put it plainly, it's an act of literary fraud, stealing another's work or idea and then lying about it afterwards. (as found at plagiarism.org)
For many reasons. Sometimes by accident and sometimes on purpose. It could also be a lack of knowledge about plagiarism, a lack of knowledge about the consequences, you might not feel confident about your own writing skills, or it could be from a severe act procrastination or laziness. It could also be a belief that it is not as severe of a problem or that it only affects the academic world or only when writing. Some people may not be aware of the proper procedures for citation and paraphrasing. (Don't worry, we'll cover some of this or provide sources that can help you.)
Overall you should know that plagiarism is wrong and you should never do it. The consequences of plagiarizing could be failing a class or assignment and later in life, it could even cost you your job.
Perhaps you've seen the "Piracy is not a victimless crime" for films or other forms of digital entertainment, right? Plagiarism is more or less the same. When you plagiarize, you are taking away the hard work and effort that someone else did and claiming it for yourself without acknowledging the other persons efforts. That is simply not fair to you and especially not fair to the one that you plagiarized as you are discrediting all of their hard work.