Fois is the liver of a duck that has been force fed. What the birds are fed is up to the producer, sometimes it's a combination of different grain, sometimes it's brandy soaked bread. There is a lot of controversy surrounding how fois gras is made, some peope feel the ducks are being tortured by going through the gravage (forcefeeding) process - I've seen these ducks happily following the farmers to get their suppers.
Opinions on feeding ducks aside, fois gras is delicious, luscious and a treat. I purchased my tiny piece at one of my local gourmet shops, it's Grade B Fois Gras, which means that there are some veins present but the flavour and texture are still fois. Just a little more high maintenance. They are fairly easily removed with a sharp knife. For those on a budget I highly suggest it.
Don't salt the liver until it's seared, then give it a generous sprinkle. The one thing to remember is to not over cook your precious liver, you want it quivering on the inside and crisp on the outside.