Linseed is the small, reddish-brown or golden-yellow seed of the common flax plant, linum usitatissimum, the same plant that's used to grow the fibre from which linen has been made for thousands of years.
Though tiny, they are rich in the omega-3 fatty acid ALA, lignans and fiber, all of which have been shown to have many potential health benefits. They can be used to improve digestive health, lower blood pressure and bad cholesterol, reduce the risk of cancer and may benefit people with diabetes.
Side-note: Linseed is a shorter plant, with lots of branches and lots of seeds. Flaxseed is taller (3 foot) with fewer branches. Therefore, linseed is good for creating oil and flax has longed been used to make linen, rope, and nets. The seeds are however seen as the same product.