Hemp is a plant. The hemp discussed here is also referred to as industrial hemp.
Hemp is a versatile crop. An online search on the topic of hemp will lead to many articles on the history and use of hemp. There are a number of books written on the subject also. Here is a link to the Wikipedia article on hemp.
In general, it is illegal to grow hemp in the United States. The Federal government has slowly begun legalizing the farming of hemp. States like Kentucky have a burgeoning hemp industry. Here in Maryland there is a UMD hemp project. Local hemp companies are acquiring hemp from outside Maryland in order to start making products.