Haven't had many ideas for posts in the last few days, but I'll put this little one up today. I'm going to explain the difference between semi-automatic and fully-automatic guns.
Semi-auto - A semi-automatic firearm is one in which you pull the trigger and the gun fires. The energy stored in the casing propels the slide or bolt back which is on a spring and goes forward when it reaches a rearward position. The magazine spring pushes a new bullet up and the slide or bolt presses it from the magazine into the chamber. The person firing the gun now releases the pressure on the trigger and pulls it again if they wish to, causing the same thing to happen.
Full-auto - A fully-automatic gun has many similarities to semi-auto, but it also much different. When you pull the trigger of a fully-automatic firearm and keep your finger on it, the gun will keep firing at a very rapid rate until the magazine runs out of bullets or you release your finger.
Fully-automatic guns are capable of much quicker rates of fire because they do most of the work themselves, requiring the shooter only to keep their finger on the trigger and keep the gun pointed in the vague direction of what they're shooting at. It's within reason that if you throw enough bullets at something you'll eventually hit it. This is generally used for suppressive fire, taking out large groups of people, or having a lot of fun just blasting away at nothing, though regulations make that hard to do.
Of course the gun-banners love to take advantage of the visual similarities of some semi-auto and full-auto guns as it increases fear and helps them work toward disarming people. For example, this full-auto M-16 rifle ( http://www.defensereview.com/stories/m16clinic/M16%20Clinic_SNIPER1-CB.jpg ) looks very similar to the civilian semi-automatic AR-15 rifle pictured here ( http://www.springsgov.com/units/police/images/AR15andcase.jpg ).
There are several design features that make it appear to be a military rifle, so it's demonized but anti-gunners. Which is a shame really, because they're not even very powerful guns.
~S