Flax is a plant, a vegetable. It is one of the richest sources of Omega 3 essential fatty acids. Flax seed is a very old 'alternative' remedy for just about anything--kind of a cure-all. But it's a dietary fiber, so it's strong points are aiding digestion but it also has disease fighting properties, and has long been associated homeopathically with helping psoriasis.
You grind up flax seed into a powder to take it. Put it on salads, yogurt, orange juice, oatmeal, you name it. It has a nutty kind of taste to it. You can buy whole flax seeds at most any health food store and grind them yourself every day.
Flax seed is a very old and well-researched supplement. Search just about anyplace that has nutrition information and they should have some information about flax seed.
I personally take flax seed as in The Missing Link. Missing Link is a product by Designing Health, and it contains mostly ground flax seed (and some other good stuff). It comes vacuum sealed, and must be refridgerated to keep it fresh. You can find out more about Missing Link and where to get it from their website: http://www.designinghealth.com/index.html