Creative Commons defines the Public Domain as work that has been identified as being free of known restrictions under copyright law, including all related and neighboring rights.

Wikipedia takes it further and defines the Public Domain as, "The legal term public domain refers to works whose exclusive intellectual property rights have expired, have been forfeited, have been expressly waived, or are inapplicable."

While the burden of determining whether an item is in the Public Domain lies with you, items in the Public Domain can be freely used for whatever purpose you choose. Items where the author has explicitly assigned a Creative Commons Zero license are considered to be Public Domain. Etiquette on Public Domain use is to cite your sources but it is not a legal requirement.

As always, the safest (and courteous) method is to cite your sources and link to the original whenever possible. Alternatively, keep copies of all documentation. This may be the only way to protect yourself in the event another claims copyright infringement.

An excellent example of Public Domain content is Unsplash.com, a site of high resolution images, all with Creative Commons Zero licenses. Their byline says it all:

Beautiful, free photos.

Gifted by the world’s most generous community of photographers.

Although their site lists all images as CC0, there are still restrictions to consider. It is always in your best interest to read the license before including works in your creations, such as the information from Unsplash.com's website below.

"All photos published on Unsplash can be used for free. You can use them for commercial and noncommercial purposes. You do not need to ask permission from or provide credit to the photographer or Unsplash, although it is appreciated when possible."

"More precisely, Unsplash grants you an irrevocable, nonexclusive copyright license to download, copy, modify, distribute, perform, and use photos from Unsplash for free, including for commercial purposes, without permission from or attributing the photographer or Unsplash. This license does not include the right to compile photos from Unsplash to replicate a similar or competing service."