A pharmaceutical medication. Chemically, it is dronabinol, which is really just a synonym for THC. It's synthetic THC basically. Not exactly weed in pill form though, since the effects are different from smoked marijuana. There are two reasons for this---
1) It lacks the other cannabinoids that occur in cannabis that add to the overall high/stone from smoking marijuana.
2) It is taken orally, so the effect is also slightly different.
Increased appetite, reduced nausea (anti-emetic), and pain relief. These are the effects most people get at the standard dosage (~20mg). However, since the pharmaceutical companies worry about people getting high off the stuff, most people never exceed the maximum daily dosage which would lead to other effects, such as---
Euphoria, relaxation, giggling, intense body buzz, altered thought patterns, and munchies (food tastes better). As stated above, since some cannabinoids found in marijuana are missing in this medication, it is unlikely that a user will experience music enhancement, closed eye visuals, or mystical experiences, which are common with weed (if enough is smoked that is). On the other hand, the user probably won't experience paranoia, panic attacks, or despersonalization/derealization. Also, since it's taken orally, there isn't the issue of lung/respiratory problems, but one could avoid that by simply cooking and eating weed.
In summary, a pretty cool pharmaceutical if you take it enough of it.
Marinol is most often given to those suffering from cancer, HIV/AIDS, neuralgia, and various other conditions. Smoked marijuana is likely just as effective.