Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. High-level and low-level are terms used in classifying levels of description and goals in many fields where systems could be described from different perspectives. A high-level description is one that describes "top-level" goals, overall systemic features, is more abstracted, and is typically more concerned with the system as a whole, and its goals. A low-level description is one that describes individual components, detail rather than overview, rudimentary functions rather than complex overall ones, and is typically more concerned with individual components within the system and how they operate. Low and high level are relative; for example the graphics engine that drives a computer game and works at the level of entities in the game, is high level compared to the video subsystem DirectX that works on the level of renderable objects, which itself is high level compared to a vertex shader within that system.