A semi-automatic weapon is not a machine gun. It is not "made to kill a lot of people quickly", as one spam I got today says.
Most pistols are semi-automatic. You pull the trigger, it shoots, and a mechanism in the gun brings the next cartridge up so you can PULL THE TRIGGER AGAIN, having spat out the now-expended first cartridge's remains (the "brass").
Revolvers do the same thing through a different mechanism, but unlike a semi-automatic pistol, they don't eject the cartridges themselves, so you have to open the revolver up and clean it out and reload after 6 shots pretty much by hand, while you can eject a magazine from your semi-auto and keep shooting -- one shot at a time.
FULL AUTO weapons are the ones that shoot lots of bullets much faster than you can pull a trigger (well, faster than MOST people can pull a trigger; some of the stunt shooters can manage an impressive rate of fire for a short time). Real assault weapons are full-auto (many can SWITCH to burst and/or semiauto, but what makes them good military weapons is that they are in fact capable of fully automatic fire.
Semi-automatic covers most handguns and a fair number of rifles and some shotguns.
Fully-automatic weapons have been illegal in most states for YEARS, and generally HAVEN'T been used in the killings people get up in arms over.