The Other files are quite simply everything 'Other' than all the rest of the categories.
The reason you don't recognize them is that most of the common things like Pictures, Music, etc, would all be covered by the major specific categories. So 'Other' really means everything else that's left over.
The reason this category is so large is that there can be many file types that don't fit one of the standard categories. In recent versions of OneCare the Other category is selected for backup by default so no potentially important data files will be missed if you simply accept the defaults. However, as you've noticed this can be quite huge, so it may be best to disable this group if you're certain you don't require any of the files it contains.
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