Before you starting downloading and using online resources it is a good idea to have a basic understanding the basic of copyright. Free access does not always mean that you can use the material in your own work.
There is an ever growing amount of free, open access music resources available online. This includes, but is not limited to, scores, recordings, teaching materials, and composition aids. Something "free" online is still subject to copyright laws.
What do they mean "open access"? When someone is talking about open access materials in an academic setting, they usually are referring to peer-reviewed research articles or books, which are free to access and use. An example of an open access source is the Directory of Open Access Journals (DOAJ).
Why can I find free works by old composers like Mozart but not contemporary ones like Philip Glass? Musical scores and recordings are protected by copyright law, but many classical composers, like Mozart and Beethoven, no longer are covered under the laws (because they died long ago); their works are public domain. This means you can freely use their compositions however you want.
But, if a person creates their own arrangement of say, the Goldberg Variations, then they can have it protected by copyright laws. That particular arrangement is not public domain! Audiovisual recordings of public domain compositions often are copyright protected, although some artists and ensembles choose to make their works public domain. Contemporary composers like Glass usually choose to have their compositions protected under copyright, and even after they die, their work still has protection for another 70 years! Their family or company may continue to retain the legal rights to their works.
Some helpful links in understanding copyright and public domain:
Media Policy and You
What is plagiarism?
When you use part of, or entirely copy another person or groups work and present it as your own original creation, that is plagiarism. In a basic sense, you are not giving credit where credit is due. Plagiarism is generally considered an ethical issue, but in some instances it is punishable under U.S. law as copyright infringement.
In an academic setting, one of the most common forms of plagiarism is failing to include citations, intentional or not. The most common citation styles used in Music are Chicago, MLA, and APA. View the Citation Guide for available citation resources.
Plagiarism is not always as straightforward as using a picture or quote without a citation; to better understand plagiarism and how to avoid it, check out these resources: