The back side of a card will be affected by both embossing and debossing processes.
Embossing will show a reverse impression on the opposite side of the stock (deboss), while debossing will show a reverse raised design on the opposite side of the stock (emboss). This is important when embossing or debossing text, as it will only be forward reading on one side of the card. See image below depicting an embossed design for text. The back side of the card shows an opposite debossed effect.
To avoid the back of the stock being affected, you can order a Layered card (2PLY) that will hide the opposite effect of the debossing or embossing inside two cards that are mounted together.
Please note that "blind letterpress" is a different process that is used to create an impression and although it is similar to a deboss, it has a more subtle effect and will not create an emboss on the opposite side. "Show through" with this process will still be visible, but the level will be dependent on the thickness of the card (for example, the "show through" will be less on a thicker/stronger stock).