Canon CRW is the original Canon RAW file, based on CIFF, a Canon image file format. They are generated by all DSLR camera up to the 10D and Digital Rebel/300D as well as the Canon G1 to G5.
There is a complete specification for the CIFF file format.
It use a lossless huffman compression.
Some camera with a "hacked" generate CRW files that are nothing like CIFF.