Standardized graphic exchange format
STEP is a standardized graphic exchange file format.
Here`s an explanation from Wikipedia`s entry for ISO 10303:
ISO 10303 is an ISO standard for the computer-interpretable representation and exchange of product manufacturing information. Its official title is "Industrial automation systems and integration - Product data representation and exchange", known as "STEP" or "Standard for the Exchange of Product model data."
...STEP can be used to exchange data between CAD, Computer-aided manufacturing, Computer-aided engineering, Product Data Management/EDM and other CAx systems.
STEP is the same as STE and STP.