Even if you have not used someone's exact words, but have rephrased their ideas you need to give your sources. The idea is that someone else reading your work should be able to recognise the difference between your work and someone else's. You need to provide them with enough information about your sources that they could find the source for themselves. There are several different referencing systems, each subject area tends to use it's own system of citations, but whatever style you choose it is important to be consistent, complete and accurate. This is matters not only for books and articles, but electronic sources as well.
Plagiarism is theft.
If someone broke into your house and stole your television you would be understandably upset. That was your television and now you can't watch The Simpsons . In exactly the same way, using someone else's ideas without acknowledging where they came from is stealing. That is what referencing is all about, making sure that you if you use someone else's words or ideas, you let your readers know where the information came from. It is okay to quote from a book, or use the ideas from someone's work in your own work, but it does mean that you need to be careful to make sure that you acknowledge where the information came from, and make it quite clear that it is not your own original ideas.