IGES: IGES is an older neutral CAD exchange format that stores curves and surfaces, still found in legacy data but now largely replaced by STEP for sending parts to be made.
IGES (file extensions .iges and .igs) was first published in 1980 as a way to move drawings and 3D geometry between CAD systems from different vendors. Like STEP, it stores exact geometry rather than triangles, so circles stay circles and surfaces stay smooth.
The difference is in how solids are held. IGES was built around curves and surfaces. Many IGES files describe a part as a collection of separate faces rather than one closed solid, and the receiving program has to stitch them together. When the edges do not quite meet, the result is gaps, missing faces or a part that will not close into a solid.
Example: a shop receives an older tooling model as an IGES. It opens as 340 separate surfaces. After stitching, two small faces near a fillet are missing, and someone has to repair them before CAM can produce a toolpath. The same model as a STEP solid would usually open ready to use.
IGES still appears in older archives, in some aerospace and automotive supply chains, and in software that never moved on. It is not wrong to accept it, but if the buyer can export STEP instead, ask for it.
Do not confuse IGES with a mesh format like STL: IGES is exact geometry, just in an older and less reliable form. Read STL vs STEP vs 3MF for which file to ask for.
Go deeper: STL vs STEP vs 3MF (and OBJ, DXF, IGES): which file to send for a quote