From what I know Tiff is just a standard uncompressed picture, where data is saved pixel by pixel. RAW however is slightly different, it also has no loss in quality AND within the file is inserted info about the contrast, saturation etc. of the picture which makes it easy to edit. It isn't a picture until it is processed further.
Damn, this is hard to explain. Let me put it this way, Tiff is basically a RAW which is saved with the default camera settings and cannot be undone.
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