The STL file format (derived from "stereolithography") has been the standard format for additive manufacturing for more than twenty years. An stl file defines the surface - and only the surface - of a three dimensional object as a triangular mesh. No other parameters are included in the standard format although certain company and/or equipment-specifc versions might include additional information such as material or color.
STL files can be exported from most 3D CAD programs. Please see "CAD to STL" for STL export directions for major programs. Also see "Tips and Tricks". to create 'print-ready' STL files. For a more detailed discussion of the STL format and some of its shortcomings see our blog post 'What is an STL file and is it obsolete?'
Although the STL format is capable it is by no means ideal. In its most basic form it does not include such information as units, materials, colors, textures etc. To address this ASTM Committee F42 on Additive Manufacturing Technologies has been working on a new format, 'AMF' intended to supercede the STL format. Like STL, this format can define a 3D surface as a series of planar triangles. Unlike STL, the AMF format can go further and define a surface as a series of non-planar triangles with curved edges.
It also includes data currently missing from the STL format such as units, colors, textures, materials, etc. It relies on whatever program is parsing the file to recursively divide non-planar, curved edge triangles into a minimum of 256 planar, straight edged triangles. Therefore for a given level of accuracy, AMF files are significantly smaller than STL files also (although read/processing times are slightly greater). For more information see Additive Manufacturing File Format. Also check out Cornell's Creative Machines Lab for a downloadable STL to AMF file converter.