Letís say youíre cheerfully writing an article, and you realize that youíve already written about the exact same situation. You pull up the old document, cut and paste the salient bits, and voila! Youíre on to writing the next section, and youíve just plagiarized yourself.
The good news is that if you wrote about it and didnít publish it or accept any money for writing it, you can use it. The bad news is that if you did publish it or accept money for it, you donít own it any more. Itís kind of like fixing up an old car and selling it. Once youíve sold it, you canít just take it for a spin if you need a car. Yes, you did all the work, but you sold it and it isnít yours anymore. Anything you wrote for an employer or under contract is wholly owned by that employer, even if it was your devilishly clever idea and implementation. They paid you to have clever ideas and implement them, and they get to keep your work and do with it as they will. You only get to point to it with pride.
You also canít take the publisherís verbal okay to re-use all or part of that earlier document. If the second publisher decides to sue for your fee or not to pay you because the material wasnít original to the new document, you wonít have a legal leg to stand on. You need to get permission in writing that itís okay to re-use text, and even an email will do. If you donít get permission, youíll get a bad reputation at two publishers because you didnít take the time to write a new piece or the few minutes it takes to get permission. (Hereís help for getting the permissions you need when the work you want to excerpt is not your own: www.iCopyright.com. iCopyright offers a centralized place to find reuse guidelines and an automated process for obtaining permission.)
Audiences for technical subjects tend to read everything thatís published on the subject. That means that they already read that other piece and you donít want to diminish its importance by using the same old stuff again. You also donít want to publish something on your own Web site and then turn around and sell it to some other publication for the same reason. Youíll lose your audience. People will stop reading your work and publishers will stop hiring you. Surely you have something new to offer?
Using other peopleís work is more complicated. Of course you know that you canít just copy someoneís work verbatim, put your name on it, and turn it in. But you CAN quote from someone elseís work, and as long as you properly attribute it, you can quote up to 10% of a printed page without getting permission (this is called ďfair usageĒ). The attribution needs to be right there where the copied text is, or asterisked and footnoted. You canít just thank someone at the end, knowing that they will recognize their own work. By the way, 10% of a printed page is about 30 words. If youíre going to use more than that, you need to get permission. If you do a lot of quoting, people will stop hiring you to write things and just go to the source.
Code is much trickier than plain old text regarding permissions. Obviously, you need to attribute borrowed code, and make note of any deviations you made to customize the code for your purpose. But the original author may have sold the rights to the code, or distributed them with caveats, and you need to check those out and comply. Thatís for all of the code, not just 10%. Because code has a function other than conveying information, you have to be more meticulous than with text alone. Otherwise itís like putting a Ferrari exterior on your í68 beetle engine and selling it as a Ferrari. Your client is going to be hopping mad when they put it in gear.
I have a theory about why thereís so much plagiarism in technical writing. When youíre in school and you write a paper, itís fair game to absorb the ideas of other writers and repeat them in your own words so long as thereís a bibliography at the end. Thatís perfectly legal and expected. But a major difference between writing a paper for school and writing one for publication is copyright. The academic stuff you wrote is copyrighted to you by default and stays yours unless you sell it. (The law says that you own it from the moment of writing it down, unless you were under contract to write it, and until it is published, when the publishing entity, by default, owns it unless otherwise specified.) It works out like keeping a journal: as long it isnít disseminated, you can quote and copy to your heartís content. But if itís disseminated, even if only a little bit, you need to play by the rules.