What is regular and secular clergy?

This is a distinction within the Catholic Church depending on whether the cleric (single of clergy) lives in isolation from the world-at-large and takes vows. For example, a Benedictine Monk who takes vows and lives in a monastery would be a "regular cleric". Conversely, a Parish Priest who lives in the world-at-large and does not take vows, would be a "secular cleric" since he lives en saeculum (i.e. in the world).