SLA machines have a vat of liquid polymer resin, and the part is "drawn" in thin layers with a laser, right at the surface. The laser cures the polymer, changing it from liquid to solid. In most such machines, the table on which ti part is drawn starts out at the surface, and submerges as the part grows upwards. The process can produce nice, accurate parts, with a resolution on the order of 0.001", but SLA machines are expensive, and the parts take a loooong time to run - run times of days are not unusual. But, they are great for making prototype parts to "test" a design, without the expense of cutting a mold.
Regards,
Ray L.