Here's the short answer:

E60xx electrodes have a 60,000 psi minimum tensile strength, E70xx electrodes have 70,000 psi minimum tensile strength.

E6010 - fast freezing puddle, deep penetration, intended for DC welding.

E6011 - similar to E6010 but intended for AC welding.

E6013 - less penetration than E6010 or E6011, good for light gage material, AC or DC.

E7018 - low hydrogen/iron powder coating, intended to prevent underbead cracking due to hydrogen pickup in the weld from moisture, iron powder increases deposition rate and makes it run smoother, AC or DC but runs much better on DC.

This may help also:

http://www.lincolnelectric.com/knowledge/articles/content/awsclassification.asp

Also, rods like 7018 are rated as such 'as long' as they're stored as recommended. Otherwise they're just another maintenance rod. Gotta keep them dry as soon as you open the can.