A J2K file contains bitmap image data in JPEG 2000 format. However, it only contains the code stream, ie the data stream that contains the pure image data.
JPEG 2000 describes both a compression method and thus a codestream (data stream) as well as a file format. While the data stream is sufficient to decode the image, the file format includes additional information such as metadata.
JPEG 2000 is a digital image compression technique that provides both lossless and lossy compression. It has some advantages over JPEG. Image quality is better at high compression rates and some limitations no longer exist. Eg image size, color depths or support for alpha channels (for transparency). However, this is offset by the licensing-related situation and a significantly higher demand for computing power.
The pure data stream (codestream) is not intended to be stored in files, so no uniform extension is required. The endings jpc, j2c and J2K are common.
There are different versions of the JPEG2000 standard, not every program supports them all. Different programs also need a plugin to handle J2K files.