OpenEXR is a high dynamic range raster file format, released as an open standard along with a set of software tools created by Industrial Light & Magic (ILM), under a free software license similar to the BSD license.
A full technical introduction of the format is available on openexr.org. The currently-supported version is OpenEXR 2.3.
EXR images are created with (1) the Save Image... menu on TOPs, (2) the
.save() python method on TOPs, and (3) the Movie File Out TOP with Type "Image Sequence".
EXR is a broad format for arbitrary numbers of channels, however TouchDesigner currently only extracting 4-channel RGBA images.