Kratom is an opiate like drug that consists of the grinded or whole leaves of mitragyna speciosa, a tree that grows in parts of Asia. The high is pretty nice for someone who's opiate naive, it is usually sedating, kind of numbs muscles, and can be stimulating. The effects depend on the individual, strain, location grown, and distributor.
The only practical way to use kratom is orally, smoking yields little to no effects. Dried and ground kratom leaf can be made into tea by boiling in acidic water. It can also be swallowed whole if capsulated or powdered fine enough, swallowing whole powder is commonly called toss and wash. Toss and wash is usually the most efficient way to use kratom but be warned it is fairly repulsive and difficult for those not used to it.
Doses for someone with no tolerance are usually around 1-4 grams of regular commercial leaf. The effects will last anywhere from 2-4 hours, usually not longer than that.
Kratom works on the opiate receptors and is therefore physically and mentally addicting. Personally I would say it's not as mentally addicting as other opiates like morphine or hydrocodone, but it is very close and still just as physically addicting.