It is essentially the same thing.
EC=Electrical Conductivity
PPM= Part per million, calculated either by ECx500, ECx640, ECx700 depending on the brand of meter you buy
TDS=total dissolved solids,
http://en.wikipedia.org/wiki/Total_dissolved_solids
You really only need to measure one of these, i prefer EC because it is a universal rate and doesnt fluctuate based on brand of meter.