Rootkit:W32/Kobchka is a Generic Detection that identifies files with rootkit-like characteristics or behavior.
About Generic Detections
Unlike more traditional detections (also known as signatures or single-file detections) a Generic Detection does not identify a unique or individual malicious program. Instead, a Generic Detection looks for broadly applicable code or behavior characteristics that indicate a file as potentially malicious, so that a single Generic Detection can efficiently identify dozens, or even hundreds of malware.
For more information about Generic Detections, see the Other:W32/Generic