NCW stands for Native Compressed Wave files. They are an audio file created by Native Instruments for Kontakt. They are designed to save an imported wave audio file using compression methods so the file uses less disk space.
NCW files are stored using lossless compression, so that there is no loss in sound quality. Additionally, the files can be converted back to the original .WAV files without loss in quality. (Here's a link to our article outlining Lossless and Lossy compression: http://help.timespace.com)
NCW files work in Native Instruments Kontakt on both Mac and Windows.