They provide more detailed information than available on the invitation itself. Pew cards are included in the invitations of guests of distinction such as close family members and special friends. Thermography and engraving are both types of raised letter printing. Thermography begins with a flat imprint of a specially-formulated ink. A powder designed for thermography is then sprinkled over the wet ink and both are bonded to the paper by heat to produce the raised look.
Thermography simulates the appearance of the more-expensive engraving process.
Engraving involves the cutting of copy or a design into a metal plate. The die is inked and wiped clean, leaving ink in the impressions. The paper is then laid on the die, and the press forces the paper into the cavities of the die to achieve the raised lettering effect. In the invitation albums I've looked through, there are different prices quoted for traditional and photo lettering. Molten lead is formed into the shapes of the letters using pre-cast molds.