KRA is the file format for Krita, a raster graphics editor. It is a ZIP archive containing a number of files, including the image data, the layer structure, and the document settings.
It is similar in function to PSD files for photoshop.
A .krz file is a compressed version of a .kra file and only missing the mergedimage.png contained within it to save storage. The lack of this file can affect interchange with other applications such as Scribus.