AC/DC welding means the two different types of polarity you will use when welding. 

Alternating Current Welding

AC means the flow of the electrical current changes direction back and fourth. See green line in image above for reference.

AC welding is a cheaper and less used than DC, that being said AC welding is great for welding Aluminum.

Pros of AC Welding

- Welds aluminum very well, Higher temperature abilities 

- Cheaper machines run only AC

- No Arc Blow

Cons of AC Welding

- Unstable Arc

- Welds normally appear less smooth

 Direct Current Welding

DC is when the electrical current continues in one direction. See red line in image above for reference.

Direct Current welding is most common for all weld processes including Stick, MIG (GMAW- Gas Metal Arc Welding), and TIG (GTAW- Gas Tungsten Arc Welding). The constant flow of current allows for a smoother overall weld process.

Pros of DC Welding

- Welds will appear smoother due to the constant current flow

- More stable arc that allows for easier welding

- Less spatter

Cons of DC Welding

- Arc Blow

- DC Welders are more expensive

- Cant be used to weld Aluminum