A serger is a special sewing machine that you can use to finish edges on garments so that they don't fray and to give garments a professional, clean look to the inside. Machine made clothing that you buy in stores have this done to the edges, and if you look inside your shirt at the top of the bottom hem, you'll see what looks like a "chain" of thread going around the cut edge. My mom has one and it's wonderful to use on fabrics, such as linen, that fray easily. They're expensive, but if you can't afford one (I most certainly can't!) a simple zig-zag stich around the edge should do the trick.
ah i see, yes i'd noticed the stitching like that on clothes. i had been using the zig-zag function on my sewing machine for that purpose too.
dontletsstart - (i think) i know what french seams are but i'm afraid i can't really explain it without pictures! i suggest you do a web search. basically it's a way of giving your seam a little hem on the inside so you don't get bits of fabric coming off all over the place.