A "ghost hunt" is when a group goes to a location with no previously reported paranormal activity in an effort to find some.

An "investigation" is when a group conducts a SCIENTIFIC study of a location with claims and reports of activity. 

Note also that many groups will conduct "group" investigations at locations where activity has been reported (Usually bars - you figure out why).  As far as we are concerned these are nothing more than glorified hunts as nothing will ever be captured beyond a wicked hangover.