EC measures the how well your nutrient solution conducts electricity... Distilled water or water w/ no particles in it does not conduct electricity at all, but the more other stuff (nutrients, calcium, whatever else is in ur tapwater) that are in the water the more electricity it will conduct... So, by measuring EC we know how much fertilizer is in the solution that we are feeding plants...
PPM is similar to EC as it's actually measuring the same thing, it stands for parts per million and will tell you how many ppm you have of total dissolved solids (stuff) in your solution... PPM is not quite as accurate as EC according to the growers bible b/c it actually takes EC and then converts to ppm... Hope this makes some sense to you??
In order to grow in hydro i suggest to get a meter, i didn't go cheap, I got a milwaukee that does ph, ppm, and ec and it cost around 185usd on ebay.. By having a meter you can know when you need to add water to lower ec or add nutes to raise ec... IMO and many others this is a very good tool to have... If you've been having great results without i would say it could only make u better because you'll know when ur plants need more nutes before they tell you...
If you have anymore questions about the ppm/ec hit me back, or maybe someone can explain way better than that...