Definition
Levels of description are different vocabularies for describing the same complex system, each capturing regularities that the others cannot. A computer can be described at the physical level (voltages, transistors), the gate level (boolean operations), the assembly level (machine instructions), the application level (programs and processes), and the user level (documents, conversations, errors). Each level is real, useful, and not reducible to the levels below or above in any simple way.
Why it matters
How it works
To choose a level for a problem, ask: at what level does the regularity live? If you are diagnosing a slow web page, the user level (page loads slowly) names the symptom; the database level might name the cause; physical-layer voltages are almost certainly the wrong level. The pattern: name the symptom at the level of observation, then move down (or up) one level at a time toward the cause, confirming at each level with appropriate measurements.