A Halbtax card (half-charge in German) entitles you to half-price travel on most public transport in Switzerland (except for the privately run ski lifts). Practically everyone living in Switzerland has one, and you can get a halbtax combined with your Visa or Mastercard. It's a worthwhile purchase if you make a couple of long trips on the SBB (they issue it).
Some auslanders call it the "non double-fare card". The Halbtax propaganda (look for large signs with 1/2 on them) claims you get discounts at places other than SBB, but I've never seen head nor tail of them. Like most of the non-Swiss, I have a rather conflicted relationship with this strange frequent-customer rewards program.
The card itself is the size of a credit card or visiting card, blue in color, with the holder's mug-shot on the reverse. The photograph is covered by the ubiquitous SBB hologram, rugged mountains with trains, ships and buses floating serenly over them.