An AMF file is an additive manufacturing file used by CAD programs to describe objects for 3D printing purposes. It contains one or more objects, which include the geometry, color, and material of the volumes that make up the object(s). AMF files also store the constellation (arrangement) of multiple objects and metadata that specifies the author, name, copyright, and special instructions for the object(s).
Since the 1980s the .STL format has been the industry standard for storing and transferring information between CAD programs and additive manufacturing tools. Now, with the rise of 3D printing, more capable formats are needed to store and transfer all the necessary 3D object information for 3D printing. This need has led to the increased popularity of the .3MF and AMF formats.
The AMF format is an open format developed by the American Society for Testing and Materials (ASTM) released in 2013. The format is designed to be a unifying solution for storing 3D object information much like the .PDF format is for storing documents. It is XML-based and was designed to be simple, scalable, and backwards and future compatible. The format uses triangle meshes for describing surfaces, whether they are curved or non-planar edges on 3D surfaces. Colors are specified using RGBA (red, green, blue, and alpha channels) values.
You can export 3D object information to the AMF format using programs such as Dassault Systemes SOLIDWORKS and Autodesk applications. 3D printers that support the format include Materialise and Shapeways. You can view AMF files with the free cross-platform Autodesk Meshmixer application.