Generally most work is DCEN - negative electrode. Most of the heat flows into the base metal causing a more narrow deep penetrating weld. DCEP (positive electrode) causes more of the heat the flow into the electrode relative to the base metal so you get a shallower wider pool. This should also help clean the weld metal, although this is more useful in TIG. This action is caused by the ion stream that you get - (electrons flow from - to +, but you also get a flow of ions from + to - to balance it out) when the ions strike the metal atoms they tend to 'knock' off things like oxides. The downside though is a hotter electrode - something like 2/3 of the heat is at the + side.
I think AC tends to be used for for various specialty stuff - what I usually use is 7014 (I think, maybe 18) with DCEN for general welding of steels. I'll use MIG (flux core) for light metal. What I recall is that 6011/13 is typically used more for out of position and field work, and 7014/18 is if you can weld it normally. I find 7014/18 to be easier to use and they lay more metal faster. The 70xx series are stronger (first 2 numbers are tensile strength in x1000 psi) but they also really should be stored dry or in an oven. Moisture is bad for electrodes period, but IIRC 7014/18 are more sensative than 6011/13.
I think AC tends to be used for for various specialty stuff - what I usually use is 7014 (I think, maybe 18) with DCEN for general welding of steels. I'll use MIG (flux core) for light metal. What I recall is that 6011/13 is typically used more for out of position and field work, and 7014/18 is if you can weld it normally. I find 7014/18 to be easier to use and they lay more metal faster. The 70xx series are stronger (first 2 numbers are tensile strength in x1000 psi) but they also really should be stored dry or in an oven. Moisture is bad for electrodes period, but IIRC 7014/18 are more sensative than 6011/13.
(No accident or property damage... just a rather amusing story for another time). 
