JBIG, also called JBIG1, is a lossless image compression algorithm and data format. The compression algorithm is based on arithmetic coding.
It is named after the organization which developed the format, Joint Bi-level Image experts Group.
JBIG is primarily used for bi-level images, but a JBIG codestream may contain multiple image "planes", which can be combined and interpreted as a grayscale image, or even a full color image.
JBIG works best when contained in some other file format. It can be stored in a file by itself, but that's risky, because the specification does not fully describe how to interpret the colors of the pixels. Reasonably-portable JBIG files are probably limited to bi-level and grayscale, and even then, don't be surprised if black and white get reversed.