The fundamental unit of data in an IGES file is the entity. There are two major categories of entities.

• Geometry entities — Geometry entities define physical shapes, such as points, curves, surfaces, solids, and relations which are collections of similarly structured entities.
• Non-geometry entities — Non-geometry entities provide specific attributes or characteristics for geometry entities. Non-geometry entities include view, drawing, general note, dimension, property, and associativity entities.

Some entities have several forms or further definitions of the entity within the entity type. Each entity is represented by an entry in the directory entry section and the parameter data section of the IGES file.