The Cardians were the servants of En and Ail, who appeared in the first arc of Sailor Moon R. Their purpose was to absorb energy from humans in order to feed the Doom Tree. Of all the monsters of the week they're probably the dimmest, lacking the ability to disguise themselves as humans or even say anything beyond their own names.
Falion collected energy indescrimitely from humans, attacking anyone it saw. It was too powerful for any of the Senshi to attack, dodging or soaking everything that they threw at it. After Moonlight Knight destroyed its wheel, however, it was weakened enough for Jupiter to kill it with a Supreme Thunder.
Hell Ant was summoned to absorb the energy of humans playing a virtual reality laser tag game. It sucked their energy out through their hair. After Sailor Moon's Tiara Action proved to be completely useless against it the Moonlight Knight destroyed it with his dagger.
The Minotauron was the second Cardian summoned by Ail. It targeted girls who were picked to be the stars of a new TV drama, and those girls just happened to be the Sailor Senshi. It drained Usagi's power, but after the four senshi regained their memories they pooled their powers and destroyed it.
Reci was given the job of absorbing energy from cherry blossom watchers. Unlike other Cardians Reci had several forms, from a pale but human looking form to a floating cherry blossom tree. She incapacitated all four Sailor Senshi before they were able to transform and even destroyed Sailor Moon's transformation brooch. Sailor Moon returned from the dark realm as, well, still Sailor Moon, but with a new brooch and wand. Despite the monster's quick speed and energy blasts Sailor Moon was able to destroy it with a Moon Princess Halation.
Vampir was the first Cardian unleashed on Tokyo, and was tasked with absorbing the energy of girls. She attacked Naru, which drew the attention of Usagi and lead Usagi to regain her memory. After transforming into Sailor Moon Usagi was able to defeat the Cardian with a little backup from Luna and Artemis.