They really do the same thing, but often times LIs are thinner, containing more water, and moisturizers contain more oils rather than emollients. I've started using some of my more watery (ie not as heavy) moisturizers (like Qhemet Burdock Root Butter Cream) as LIs on wet hair and they work awesome. Sometimes I use heavier LIs (like Cantu) to moisturize dry hair before bed.
Note though - I'm natural and air dry so I wouldn't know if this applies if you are relaxed and blowdry...