What is the difference between Hot Foil vs Cold Foil?

Hot Foil -  is an opaque layer of foil placed on the card after printing is completed. No shading or gradients in it - solid, metallic, flat color.

Example: Your company has a golden star as a logo/icon and you want to produce it with a gold shimmering foil.

Cold  Foil - A layer of metallic film is applied to the cards prior to printing and then ink over it to give it metallic look in the areas you want metallic. may be shaded, have gradients, has depth.

Example: Your company has a rainbow with butterflies as a logo and you want all the elements and all the shades of color to be shimmering and metallic.