Plagiarism or referring?

Newton said he stood on the shoulders of giants. Or was it Einstein? Actually, it holds for both. Everything we write is based on what we have learnt before, and many ideas are taken from elsewhere. The honorable thing to do is to attribute the idea to the ones who generated them, or at least acknowledge that it is not your own idea. In fact, it usually reinforces your point of view if you can attribute parts of it to others: it means you are not the only one thinking crazy thoughts.

Several tools for searching in a document whether it was original exist, apart from the ubiquitious google.com:

http://Turnitin.com supports web and Ipad app.
http://plagiarisma.net supports web and Android and IOS apps.