The term 'citation' means 'the practice of citing sources'. It also means: 'a reference to a source of information, especially in science'.
Two kinds of citations are used in academic publications: in-text citations and full citations.
Citation searching is a search method that you can use if you have already identified a key publication (a highly relevant publication on your topic).
There are two kinds of citation searching: backward and forward.
FORWARD citation searching is identifying newer publications on the same or similar topic that cite your key publication. These are called citing publications (i.e. publications citing the key publication).
Here's a graph that visualizes the citation relationships between the key publication and its cited & citing publications:
Google Scholar tracks how many times an item is cited by other documents in Google Scholar, such as articles and books from Google Books.
It's really very simple. Clicking on a 'Cited by' link ➊ will display a list of publications from Google Scholar that cite your key publication. To view citing articles from the library database Web of Science ➌, click on the link of the same name.
Clicking on 'Related articles' ➋ will get you a list of articles ranked by the number of shared publications listed in the reference list. This ranking is based on the assumption that articles which have a large percentage of their cited sources in common must be discussing similar topics.