they all work by building in layers, hardening some material and then using something else for a support structure (some use the unhardened building material, some use a weaker (or soluble) building material, some use the same material, in a thin lattice formation).

Its been several years, but I used to work at a prototyping company. They have a decent overview of the machines they use, which cover all of the major technologies: http://www.rpc.msoe.edu/machines.php