That really depends on the film, and on what you're looking for.
Usually, "unrated" implies more graphic footage - graphic in terms of language, sex and nudity, or violence.
For example, in "American Pie", the unrated version has slightly longer footage of Shannon Elizabeth touching herself, a different angle for the infamous pie scene, and a few more pages shown from the sex manual. Those were the only differences I noted.
In the case of "Robocop", the Unrated Criterion DVD is much more violent than the Orion/Image (?) R-rated theatrical version release. This is very noticeable in the scene where Murphy is gunned down.
The most common films with unrated and R-rated versions, however, are the B-movie softcore flicks, like "Emmanuelle: First Contact", "Secret Games 3", and the like. WARNING
: For those into these types of films (present company included... sometimes...
), the DVDs of both these titles marked "unrated" actually contain the R-Rated versions. The versions that came out on original VCD and on laserdisc had more "interesting" footage.
I learned the hard way...