A PLY file (Polygon File Format, also called the Stanford Triangle Format) stores 3D geometry and, crucially, a colour value on each vertex. That makes it the natural format for 3D scans and photogrammetry, where colour captured from a real object is baked into the model. You can open one in the PLY viewer with no software.
Here is what a PLY is and where you will meet it.
Geometry with colour built in
Most exchange formats keep colour in textures or side files. PLY takes a different route: it can attach a colour directly to every vertex of the mesh. When a scanner records the surface of an object, it records the colour at each captured point too, and PLY stores both together. Open a typical scan and you see the real colours along with the shape.
The format of scans and photogrammetry
If you have used a handheld 3D scanner, a photogrammetry app that builds a model from photos, or a LiDAR capture, the output is very often a PLY. The format suits this work because it can hold either a finished triangle mesh or a raw point cloud, and it carries the captured colour without needing separate texture files.
Point cloud or mesh
A PLY can describe a connected mesh of triangles or just a cloud of points with no faces joining them. Fresh scan data is frequently a point cloud, which is later processed into a watertight mesh for printing or rendering. Both are valid PLY files; they simply differ in whether the points are stitched into surfaces.
A note on size
Because scans capture so much detail, PLY files can be very large. If yours is heavy or slow to open, the guide on reducing a 3D model’s file size explains how to simplify the mesh without wrecking it.
Open and inspect one
To look at a PLY, drop it into the PLY viewer. You can rotate, zoom and pan, switch to wireframe to study the mesh, read the dimensions and triangle count, and save a screenshot, all in your browser, with the file staying on your device.