They are fairly simple to use and are said to make excellent coffee. See Sweet Maria's Vacuum Coffee Brewers for more details. These are sort of a reverse espresso machine. They have two separate chambers. Water goes in the bottom, coffee packed between the two, and the top chamber will contain the brew when done.
Heat is applied and the heated water is forced up through the coffee into the upper chamber.
This makes a very strong coffee. They are not much more than a small copper pot with a long handle off one side. Very finely ground coffee is placed into the water and the pot is heated over a flame. They work well for backpacking as they take up little room and the handles are removable. To get the best out of any of these methods I still recommend roasting and grinding at home.