This is a common question and causes a lot of confusion. Hemp is actually still cannabis but it is of different genetic stock. Cannabis/marijuana has a higher THC content than hemp and typically hemp has a THC content of lower than .3%. It may be slightly higher, but it would never get you stoned. Hemp does however contain cannabidiol (CBD) and other cannabinoids. Hemp can be bred for industrial uses such as clothing or for medicine. Cannabis/marijuana and hemp are typically cultivated in a different way. They also have different legal statuses depending on where you are. There is a lot of debate about whether hemp can offer the same medicinal benefits as cannabis/marijuana. Most people in the industry seem to agree that the whole flower plant from cannabis/marijuana with THC and a full spectrum of cannabinoids offer a much more powerful medicine. That said, hemp is a very valuable crop which can translate into so many different material goods that most people that are pro legalization for cannabis/marijuana are also pro legalization for hemp.
I hope this helps!