Snowball method
For many publications you will find a list of literature (or bibliography) at the back. Here the author lists the literature used. You can use this literature list to find more related and interesting sources. A disadvantage of this method may be that it does not allow you to find recent information.
Citation method
The quotation method works exactly the other way around. If you have found a relevant article you can see which other sources refer to this article. In this way you will find more up-to-date literature on the subject.
In the example below you can see 'Cited by' in a Google Scholar search. When you click on it, you will find (newer) articles by authors who have used the article (also useful: the option 'Related articles' for finding articles on the same subject).
Summary:
The snowball method is a backward reference search (searching in the past) and the citation method is a forward reference search.