Classification vs. Keywords
Patents are classified into different categories, making it easier to find similar inventions without having to think of keywords to describe them. Searching by classification is more efficient because it is:
The Cooperative Classification System is one commonly used system.
For example, a patent for Gaze Detecting Heads-Up Display Systems (US2013222235) is classified with 3 CPC codes, one of which is:
The class G02B27 covers optical apparatus; the next level down, 0172, refers to head-up, head-mounted systems characterized by optical features.
It's a good idea to start searching for patents with keywords, then make note of the classifications applied to patents you're interested in and search using those. In espacenet, there's a classification search screen with helpful tips in the sidebar.
Classification codes are just one piece of information in a patent. This excellent guide from Queen's University librarian Michael White outlines other key parts of US patents.