If something is considered common knowledge, then it does not need to be cited and/or included in a reference list. Sometimes it is not clear if something is common knowledge or not - therefore if in doubt, cite it! You won't be penalised for citing something necessarily, but you could be penalised for not citing when you should have.
Common knowledge is things that most people would be expected to know. Things like historic dates, geographic information, names of companies, etc. Think about if it can be found in many many sources - if so, it is probably common knowledge.
Examples of common knowledge include:
Is everything on the internet common knowledge? NO. Whether you found the information online or in print, you must cite your sources unless it is obviously common knowledge. Googling information and including information in your article that you found from browsing the internet without citing it is considered plagiarism.
Are things in newspaper articles common knowledge since everyone can read them? NO. Whiles newspapers do of course contain facts that are common knowledge, not everything in a newspaper article is a fact. Newspaper articles contain the author's original research/investigations, as well as possibly their opinions, and so should be cited properly.