From the Catholic Encyclopædia: "By martyrology is understood a catalogue of martyrs and saints arranged according to the order of their feasts, i. e., according to the calendar."
Liturgically, the Martyrology is read in the Divine Office at Prime, always anticipated, that is, the reading for the following day is read. With the suppression of Prime in the Novus Ordo "Liturgy of the Hours", which replaced the Breviary, following Vatican II, the book is not used liturgically in the new rite, although the custom exists in some places, including the United States, of reading the entry for the birth of Jesus before midnight Mass on Christmas.
While only required in public recitation, it has always been considered praiseworthy for individuals reciting the Office alone to read the Martyrology at the appointed place in Prime.
Even if not used liturgically, the Martyrology is a valuable resource. Literally thousands of Saints are listed in order of their Feasts or Heavenly Birthday and for those whose names are known only to God and are commemorated on All Saints Day, each reading ends with the words, "And elsewhere in divers places, many other holy martyrs, confessors, and holy virgins. R. Thanks be to God."
The Roman Martyrology.
The article from the Catholic Encyclopædia.