Owl,
DC welding makes you trigger an arc from-to instead of alternating as in AC.
To explain,
DC- :current flows from - to +, with the ground clamp being + the arc goes to the workpiece and melts the workpiece more than it melts the rod.
Deep penetration and more weld per rod length.
DC +: rod attached to + , groundclamp to -, arc goes from workpiece to rod, melting the rod faster / more than the workpiece resulting in less penetration and more material deposit. used for thin materials, to avoid warping and heat sensitive combinations (eg 2 metal types with big expansion differences) to avoid internal stress build up.
It in other words gives you a heat direction : to or from rod. whereas the AC just alternates between rod and workpiece and leaves you with less control of the weld. Also voltage and current settings are different for DC and AC welding to get to similar results.
In the more complex welders (DC exists in both rod and wire welding) you can set timing of AC, dc+, dc- and in automated systems even the switching between the modes (strike arc in ac then switch to dc) etc