Every patent includes a list of References Cited. This is usually a list of related patents and sometimes includes references to journal articles or other scholarly literature. When viewing the HTML version of a patent, the patent references usually appear as links so you can easily click through to read the references.
Similarly, the HTML version of a U. S. patent includes the Referenced By link to all of the patents that cite the patent you are currently viewing.
If you know the patent number of a technology of interest and the patent is dated after 1976, a keyword search for the patent number will also return all the patents which cite your patent.
Citation searching is an excellent way to track trends in a particular field or technology.