Kalien,
A good question. It certainly would be great if everyone shared your concern and, I might say, consideration. They don't, obviously, as some of Lisa's previous comments about people stealing her stuff indicates.
There is an old saying: "Steal from one person and it is plagiarism. Steal from two people and it is research." It is not really that easy but there are some rather basic guidelines.
Basic no no's include: repeating someone else's words directly (obvious), repeating someone's points (the same points, the same order), imitate the same flow of ideas. You've heard people say that they can't define something exactly but they know it when they see it. If you practice good judgement here you should be in good shape.
If you study any idea in several sources you will surely come to conclusions that are your own. If you think them over you will organize them in patterns that are your own. If you express an idea that is a specific thought that you find in only one place you can always give credit. (For example, I often quote the line: everyone is ignorant, just in different subjects. That comes from Gerald Mann and I have never heard it from anyone else so I always give him credit.)
You can find an excellent discussion of the subject on a professional level by going to
http://owl.english.purdue.edu/owl/resource/589/01/
Good Success!
James