RK is a proprietary compression format introduced by RK Software (later M Software). It is the native format of RK Archiver (later WinRK). Initially, there were three versions of the format - RK Archiver, RK Audio and RK Image. In later versions, these appear to have been consolidated into a single codebase.
This has no connection to a different WinRK, which appears to be a UK-based database of ringers... of bells, probably?