I am assuming you mean Ancestry. If that is the case, then Ancestry itself is not a source, but is a repository of source databases and user generated information. So, how you cite information you found on Ancestry will depend on what the actual source is.
A source is defined as a a book or document used to provide evidence in research - and in genealogy, we have two types of sources, primary and secondary sources.
A primary source is something that provides direct or firsthand evidence about an event, object, or person. These can come in the form of historical and legal documents (Birth, Marriage and Death Records), statistical data (Census records), eyewitness accounts, etc.
A secondary source is something describes, discusses, interprets, analyzes, summarizes or otherwise process information from a primary source. The can come in the form of newspapers, book reviews, or any other document that relies on a primary source.
There is also tertiary sources, but IMO should never be used in genealogy unless the primary and secondary are unavailable and the source is clearly marked as being an assumption. This includes items like chronologies, abstracts, and Ancestry trees that do not link or contain primary sources, or are built solely on secondary sources.
For more information, see the Help:Sources page for examples on how to cite common sources.