The bumps you see on a deck of standard Bicycle playing cards (and on many, many other decks) is texture added as a part of the finishing process of the cards. These bumps are actually small dimples on the surface of the card. They are designed to reduce drag between playing cards. This is the same idea as a granton edge on a knife or the dimples on a golf ball. These dimples allow for the cards to slide across each other without sticking nearly as much. If you ever find a deck of cheap, plastic cards without these dimples, chances are that they will stick together and won't fan, spread, or deal as nicely. The reason that you can do these things easily with a deck of Bikes has a lot to do with this textured finish. Another advantage for these dimples is that they trap dirt and oils in the valleys of those ridges, which allows the cards to spread and slide smoother for a longer period of time.
The easiest way I have found to write on a playing card with this kind of finish is by using a marker (usually a Sharpie) instead of a normal pen.
I hope this helps. Have an awesome day!
// L