Academic institutions, whether middle schools, high schools or universities, tend to have strict antiplagiarism rules. In general, the higher the institution, the more strict the rules.
Universities almost universally fail plagiarized work, and sometimes pursue matters further, investigating your old work and occasionally asking you to continue your studies elsewhere. The bare minimum -- failing an assignment or course -- is bad enough, as it represents a loss of time and money and a lower grade than you could have gotten without plagiarizing. Expulsion is even worse, as you will have to find a new institution to study at and you will likely lose money due to cross-crediting policies.
Plagiarizing other peoples' work in other contexts can have similarly dire consequences. If you submit a plagiarized piece of work to a magazine, newspaper or other publication you may get away with it. Alternatively, though, you may not, and they could demand their money back. At the very least, they will have to print a retraction and will probably never work with you again. Your reputation among other publications will also suffer as a result of this.
Whenever you read something, you are placing some trust in the author. After all, you are letting him tell you something, and while you may not agree with it you are at least taking the time to read what he has to say. If someone plagiarizes, though, they are violating this trust -- if you're willing to lie about where you got your information or who wrote your work, you are likely willing to lie about other things too. Plagiarism, then, undermines readers' natural trust in writers and ultimately affects everyone.
Most importantly, though, plagiarism is simply unethical. Whether you get caught or not, you are telling a lie when you submit plagiarized work. This means that anything you do after the plagiarized work is not based on your efforts, but rather was based on your ability to cheat in the past. It also gives you a dishonest edge over people who chose not to plagiarize. They may not have had the time to generate work as good as yours, as it is substantially easier and less time-consuming to plagiarize than it is to think of, draft and write your own ideas.