If you want to use copyrighted material in work you are creating you generally must obtain permission of the copyright owner and you may have to pay a royalty for the privilege. When negotiating for use be sure you understand the extent of permission you need and are being given.
For example, a magazine publisher may want to be the first to publish your work and will use it only once. Therefore, it might negotiate to buy First North American Serial Rights. If the publisher wants more extensive rights it would expect to pay more for greater use of the work. A nonexclusive license, permitting the owner to sell an article to others as well will cost less than an exclusive, which denies the owner further income from the article for a period of time or indefinitely.
The Copyright Act includes the "Fair Use" doctrine which permits limited use of material without permission for educational, scholarship, research, news reporting, criticism or comment. In determining whether a use falls under the doctrine courts look at the nature of the use (whether it is commercial, for educational purposes, etc.); the nature of the copyrighted work (is it published or unpublished); the amount of material used and its significance to the copyrighted work; and the effect the use will have on the market for the copyrighed work. This is an unsettled area of law and, before using any material without the owner's permission you should seek legal advice. There are no hard guidelines on what is a fair use. You can't say, for example, that if you use 100 words from someone else's work you will be covered by the Fair Use doctrine.