There are three main types: domestic, professional and overlocker. I have one of each. Professional ones sew really fast but can normally only do straight stitches. Domestic ones can be adapted with a walking foot: without it, the feed dogs that move the fabric along can only catch the bottom layer, but with this attachment, both layers are moved at the same rate. This is useful to prevent puckering and great for joining lots of layers. Overlockers result in a stitch that you can see on the inside of T-shirts. They sew, neaten and cut excess fabric off at the same time. You have to thread them with tweezers!
The Cycling Curtain Maker 