The Hemp plant can be used for a number of useful things such as, fibre production, biomass diesel (it is one of the best biomass plants).
It is related to, but not the same as, marijuana and doesn't produce and noticable quantity of psychoactive substances. Its relationship to marijuana has however meant that its growth in most first world countries is outlawed (or very tightly controlled).
Much of the campaigning against industrial hemp (in the USA) was by the president of the DuPont corporation, whose business was under threat by hemp's benefits over synthetic materials. It must be noted that the DuPont corporation did produce some hemp based products.