The Industry Foundation Classes define a schema for exchanging Building Information Models (BIM). The IFC definitions are documented using EXPRESS, the same language used by the STEP models. Many of the geometric definitions in IFC trace their origins back to STEP.
The IFC files use the same Part 21 file format as STEP. The contents are just described by the IFC EXPRESS schema rather than a STEP schema.
The schema below is IFC4, which is the current version. Earlier IFC 2x3 and IFC 2x2 are still occasionally used. There are structural changes between the versions which can be automatically handled by STEP Tools libraries.