In Star Trek Online, Zen is the method of currency used to purchase items from the Cryptic Store. They can be bought from the Perfect World website using real world money. An alternative method of gaining Zen is the Dilithium Exchange, at which refined dilithium can be traded for Zen with other players at a player-determined rate (kept within a range of 25-500 dilithium per Zen). One can also gain Zen through Arc platform by doing Arc Quests, where Arc points can be exchanged for Zen.
This real-world currency was previously called Cryptic Points (February 2010 - February 2011, August 2011 - July 2012) and Atari Tokens (February - August 2011), before being renamed Zen. The conversion rate was $1.00 USD = 100 = 80 .
They have a variety of functions, including purchasing races, ship classes, and costumes. It is not uncommon for 'free to play' games to sell equipment that gives buyers an edge over other players for real world cash in order to fund the game, and most 'pay to play' games have a system where players can buy aesthetic items, name changes, server transfers, and other equipment and services which don't directly affect game play. In some cases, zen are the only way to access many ship classes, such as the NX Class Light Escort or the Oberth Class Light Science Vessel.