In modern usage, although referring to members of the Cannabis family, there is a distinction between hemp cultivated for psychoactive substances (marijuana) and those cultivated for fibre and seed production (industrial hemp). Its relationship to marijuana has however meant that its growth in most first world countries is outlawed (or very tightly controlled).
Hemp as Fibre
Hemp fabric is woven from the bast fibres of the plant (the inner bark of phloem of the stem) that is seperated from the woody core (xylem) of the stem through a process called retting. The retted fibres are then spun and woven into fabrics or used in other items such as cordage. It is generally considered difficult to tell bast-fibres (including hemp, linen and nettle) apart by eye, or even under a microscope, so it is difficult to find archaeological items that have been positively identified as hemp.
As with other bast-fibre fabrics, hempcloth is stain resistant because dirt cannot penetrate far into the fibres, but simultaneously this property makes intentional staining, such as dyeing difficult.
Hemp as Food
Mention of hempseed pottage is in the 15th century cookbook of Martino of Como, and hempseed oil was used in Russia as an alternative to animal fats on fast days (see Bread and Salt p.5).
Hemp and Nettle: Two Food/Fiber/Medical plants in use in Eastern Europe. 
Bread and salt: a social and economic history of food and drink in Russia. GoogleBooks Preview [http://books.google.com.au/books?id=DKw8AAAAIAAJ
The Art of Cooking by Maestro Martino of Como. GoogleBooks Preview