Farmers walk is a strongman move where they cary anvils or other heavy objects for distance, in a gym yeah you just get some heavy ass db's. Dont hunch your shoulders up, let them drop down and relax but keep a good posture at the same time. Just walk up and down the gym. Big_BK has some sick farmers walk videos on his youtube channel...
I do it a bit lighter than most, strongmen do like 30-60ft idk how far that is actually but last time I used the 50lb DB's and went the length of my gym 6 times, Im gonna start doing them heavier now though, I had great forearm and grip results from doing this and it burns your traps, just dont use wrist wraps!! (my forearms have grown a little over 3 inches since september 08, in my avatar they are 13.5 inches)
Oh and you asked what areas it works, its a full body move really but most people consider it a trap/forearm exercis, give it a try!