A double cam bow has 2 cams that are identical in size ,shape,and form.They work in unison as the bow is drawn back.They both share the load.
A single cam bow is one with only one cam on the bottom and an idler on top.The idler being round,and the cam on the bottom is different looking then the top,except on few models.
Single cams in general are easier to draw and have more of a valley,but dual cam bows are supposed to be more efficient making them faster in general.
I hope this helps a little and maybe someone more technical will step in and answer as well