Shot my AR this past weekend and experienced a continuous two shot burst.  I've loaded the mag with three rounds as usual. The first shot went off normally but when I squeezed the trigger the second time two rounds went off continuously emptying the mag.  It seems to functioned normally after that.  

Is this what's considered a slam fire and what exactly cause it.  Thanks for any info you can provide on this.

It's possible, but probably not a slam fire.  More likely the disconector just dropped the hammer.  If it continues to act up, I would pull the disconector out and shave about .030" off the bottem of the front foot and it should solve your problem.

Seconded. Likely you have some worn parts.

A slam fire is where the primer is fired by contact with the bolt, usually from a high primer or the like. Thing is that the AR15 closes the distance from chamber shoulder to bolt face (where it would initiate a slam fire) long before the bolt is rotated and locked. If you had a slam fire, it likely would have generated big pressure long before the bolt locked, resulting in a burst case and possibly worst.

Look at your trigger parts real close. I suspect that new ones are in your future. How many rounds on the rifle?

Don't modify your trigger, hammer or disconnector unless you absolutely know what you are doing with trigger mchanisms.  This is not a procedure for an amateur.  The safety of your gun is at stake. it onlytakes a couple of thousandths to make the gun dangerous & unreliable.
Just get a new disconnector & hammer and sleep peacefully while at the range hodong your loaded BUT SAFE weapon.

A couple things you might try prior to any disassembly. First perform a function test. If this is a new weapon most owner's manuals explain a function test. If not the generic for AR clones is as follows.

Clear the weapon. Remove the magazine, inspect the magazine well, inspect the chamber.

Ease the bolt forward, place safety in the safe position. Press the trigger. Nothing should happen. Remove your finger from the trigger, put the safety in the fire position and then press the trigger AND HOLD IT TO THE REAR OF THE TRIGGER GUARD. The hammer should drop. While holding the trigger to the rear, cycle the charging handle. The hammer will cock and should remained cocked. Now ease the trigger forward until you hear an audible click. This is the disconnector releasing the secondary sear and the primary sear engaging. Press the trigger again. The hammer should fall
If the weapon passes the function test, the most common reason for the scenario you described is improper trigger control.

When you press the trigger to fire the weapon you should actually hold the trigger to the rear of the trigger guard and then allow it to travel forward just enough to allow trigger reset (the transition from the secondary to the primary sear). The double fire you described occurs most commonly when a shooter relaxes his finger immediately after trigger break. Even with the slight recoil of a 5.56, there is some reward movement of the weapon. As the weapon then moves forward from the pressure of the shooters shoulder, the trigger comes in contact with the trigger finger "floating" in the trigger guard. Resulting in a supprise second shot.

As posted earlier, true slam fire are very rare in ARs. Discharges as the result of a foreign body on the bolt face almost always occur whit the bolt in the unlocked position. If this happens you will be aware it is not simply another discharge.

In addition, discharges as the result of a secondary sear failure are almost impossible. If the secondary sear fails to engage and capture the hammer during the reward travel of the  bolt, the hammer will follow the bolt forward but will not strike the firing pin with enough force to discharge the weapon. We have in fact intentionally ground the disconnector off to demonstrate this point.

As for unintentional discharges as a result of trigger, hammer, disconnector problems. The most common occurs when excessive wear occurs in the primary sear engagement. The first round goes off, the bolt cycles, and the secondary sear engages. The bolt returns to battery and when the trigger is allowed to travel forward the secondary sear drops, the primary does not engage and you have an unintentional discharge. If you had a true unintentional double fire this is the most common cause. Once again, the function test is intended to check for this condition.

Hope the information is useful.

dws1057,

The improper trigger control seems to be a valid reason for the double fire at this point since the rifle passed the function test and only occurred once.

As I recalled, this happened when I was doing some standing shooting where light trigger control was applied and resting the stock butt along side on my shoulder (instead of against it as you would with bench shooting.)

You point of the relax trigger finger after the first shot and the rifle recoil movement could have caused contact with the "floating" finger thus setting off the second shot.  The lighter springs set for the fire control could also have contributed to the situation.

Now, only if I could capitalize this technique and have two shot burst on demand.

You just bump-fired it.  Practice, and you will be able to do it on demand.  It's not easy from the shoulder, though.