It's also known as Länder Ticket ("State Ticket") and is good on all trains in NRW (North Rhine/Westphalia ) after 9:00am -- except IC/EC/ICE. The standard one is good for up to five people and is a great deal if traveling in a group. There is also one good for just one person. I don't know current prices but at one time I think it was around €18 for the single, €24 for the group. Probably higher now, but not a lot more. Oh, and its good on most city transit systems as well.

I've bought in the station kiosks, but sometimes I had trouble finding it - -but that is probably just me You can buy it at the counter but they charge you a bit extra in that case.

Other states have their version -- I use the Bavarian one ("Bayern Ticket") quite a bit when I'm over that way.