OBJ: OBJ is a text-based 3D mesh format from computer graphics that stores vertices, faces and texture coordinates, and is sometimes used to send colour models for 3D printing.
OBJ was developed by Wavefront Technologies for animation software and became a common way to exchange models between 3D graphics tools. It describes a mesh: a list of points (vertices) and the faces that join them. Unlike STL, it shares vertices between faces, can use polygons with more than three sides, and can reference a separate material file (.mtl) and texture images for colour.
That colour support is why OBJ turns up in 3D printing, mostly from 3D scans, game assets and artistic models that will be printed in full colour.
Example: a buyer sends a scanned figurine as figurine.obj, figurine.mtl and a texture image. If the shop receives only the .obj, the shape arrives but the colour does not.
The traps are the same as for other mesh formats, plus a few of its own. OBJ has no units, so the buyer has to say what size the part should be (see file units). Models from graphics tools are often not closed solids: they may have open edges, internal faces or zero-thickness surfaces that look fine on screen but cannot be printed. And because it is text, a detailed OBJ can be very large.
For printing, 3MF carries colour and units more reliably. Read STL vs STEP vs 3MF for a comparison.
Go deeper: STL vs STEP vs 3MF (and OBJ, DXF, IGES): which file to send for a quote