'Third party' copyrighted content refers to material that is not your own work, or which is not in the public domain. It may also include work that is your own work but for which you do not hold the copyright because of an agreement you have entered into with another party, such as a publisher or your employer.
It does not matter where the item is sourced from - whether it be from a print or physical source, or from online - copyright will still apply. Duration of copyright is usually 70 years after the death of the creator.
Some things to consider when using third party content: