When you start out with citing and referencing it's easy to get confused as to which is which:
You do not need to acknowledge a source for information that is common knowledge.
Common knowledge is information that either the general public or the average reader in your subject area would know. If you are unsure if something is common knowledge you should include a reference for it.
For detailed information with specific examples and a link to the full length version of DkIT's Guide to Harvard Referencing see:
Quick guide to Harvard referencing.