STEP is the informal name for ISO 10303, a family of standards for representing and exchanging product data. Its scope is broader than older geometry-focused formats such as
IGES, because STEP was designed to support a wide range of product information across the lifecycle, not just geometry.
In day-to-day CAD usage, an STP file is a Part 21 STEP physical file, defined by ISO 10303-21. It is a clear-text format for data that conforms to a schema written in the EXPRESS modeling language. The file has a mandatory HEADER section, which identifies the file and its schema, and one or more DATA sections holding the model as a sequence of typed entities and references.