The term "public domain" refers to works in which copyright has expired.
For example, although the copyright in Shakespeare's plays expired long ago, many of the published editions of his plays contain added original materials (such as footnotes, prefaces etc.) which are copyright protected because the authors have used skill and judgment in creating the new material. This creates a new copyright in the added original material, but not in the underlying text of the original work in which the copyright had expired.
And don't assume that everything you find on the internet is in the public domain just because it is publicly available. Most of the material you find online is protected by copyright, however, you may nonetheless be able to use it for educational purposes because many uses will be covered by fair dealing or the exception for educational use of material publicly available through the Internet. See question 3.6 for further information about using material found on websites.
Note: Some copyright owners have made clear declarations that certain uses of their copyright works may be made without permission or payment. The Reproduction of Federal Law Order, for example, permits anyone, without charge or request for permission, to reproduce Canadian laws and decisions of federally-constituted courts and administrative tribunals in Canada.