What is the Foil cards ??

foil cards make foil badges, of which there is only one level.

normal cards make normal badges, of which there is five.

foil cards are typically rarer, and thus more expensive.

Foil cards are more rare and hence have a higher value. You can also craft them into a badge, which afaik only has one level instead of the usual five but gives you 500 xp ;)

Ok Thanks for replaying ;)

Yep. Foils are rare versions of cards. I think they are 1 in 100 - in couple hundred...