The trickiest part of these rules is the part about giving credit to the work of others. This is tricky for American students as well as international students, and sometimes even for established scholars. (Consultants and journalists, by the way, often violate these rules; some high-profile journalists have lost their jobs because of plagiarism!)
The basic idea is that if you quote, or paraphrase, or use key ideas from someone else, you must report this in your paper or presentation. This applies to words, major arguments, unique concepts, as well as (importantly) to tables, graphs, maps, and figures. For direct, word-for-word quotes, you must put the copied words between quotation marks; if you used your own words to express the idea or information in a source, then no quotes are needed, but you must still cite the source. The one exception is when the information is generally known or common knowledge. Common knowledge is information your readers will already know, is found using general reference sources and is undocumented in at least five other sources.
Neglecting to give credit when it should have been given is called "plagiary," from the Latin word for "stealing." So, if you do give credit, and use quotation marks when they are called for, then using information from another source is not plagiary and is perfectly legitimate in your paper.
Needless to say, it is sometimes hard to determine when there is plagiarism or not, when it was intentional or due to sloppy writing, and when it is major or minor. Given this uncertainty, it is better to be safe than sorry and to avoid falling into plagiarism if you can avoid it. There are also different ways to cite the work of others, depending generally on what and how much you copy.