Sacred music is music associated with religious or spiritual worship. It differs from secular music not in terms of the music itself but through having religious subject matter. In almost all Christian sacred music, the subjects are taken from the Biblical Old or New Testaments—or from closely related apocryphal stories that have a Biblical basis or connection.

Secular music is music that does not primarily have a religious subject, though it can mention the divine or holy. Its emphasis is on ordinary, everyday content: love, work, nature, grief, and folk stories. Secular and sacred types of music both run the gamut in emotional tone from somber to joyous.

Oratorio and opera are examples of musical forms that are very similar, although oratorio is sacred and opera is secular. Opera arose from the desire to expand oratorio during the Renaissance to encompass secular subject matter. Opera, like oratorio, adopted instrumentation and vocal singing to tell an often dramatic story. In the case of oratorio, the subject matter is sacred: Handel's oratorio The Messiah, for instance, tells the sacred story of Jesus, while an opera, such as Handel's Giulio Cesare, tells the secular story of Julius Caesar's conflict with Cleopatra, a political and military tale. As can be seen in the above example, the same composer can write in both modes, and it can be impossible from the music itself to tell if it is sacred or secular—it is the subject that decides into which category the work slotted.

Sacred music fixes our thoughts on God or the divine, while secular music occupies us with the earthly.

It can be hard to distinguish secular music from sacred music by simply listening to it. The difference is in the purpose of the piece of music and often in the lyrics, which in sacred music glorify God (although instrumental sacred music exists as well).

During the Middle Ages (before that we don't have as much knowledge of musical history as sources are lacking), sacred music prevailed, due to the fact that the Catholic Church was extremely dominant. Originally monophonic (one instrumental/vocal line at a time), it then evolved into polyphony by the twelfth century. In the meantime, secular music developed as well, especially in France, where the troubadours flourished. The theme of sacred music was the love for God and the worship of God in general, whereas the French troubadours sang of courtly love and chivalry.

Many great composers were not strictly dedicated to sacred or secular music, but wrote both (for example, Mozart wrote a Requiem but also a number of symphonies, sonatas, and other compositions).

Today, secular music is anything from pop music, to jazz, to soundtracks. In other words: anything not intended to have religious purpose.

Secular and sacred music differ only in their intended purposes. These categories are most often associated with Western culture. Music is said to be sacred when its stated intention is for religious purposes. It does not necessarily need to be used for worship, but it is most often related to a specific religion. Christianity, and more specifically, Catholicism, has a long and rich history of sacred music.

Some excellent practitioners include but are not limited to Hildegard von Bingen (Symphonia armonie celestium revelationum, which includes responses, antiphons, hymns, and sequences), Gregorio Allegri (Miserere, a setting of Psalm 51), J. S. Bach (Mass in B minor, a musical setting of the complete Ordinary of the Latin Mass), Handel (The Messiah, an Easter oratorio), Mozart (Requiem, a funeral mass), Haydn (Missa in Angustiis: Mass for troubled times), Beethoven (Mass in C, a musical setting of the complete Ordinary of the Latin Mass), Brahms (Requiem, a funeral mass), Verdi (Requiem, a funeral mass), and Poulenc (Gloria, a setting of the Gloria text from the mass ordinary).

Sacred music also encompasses genres such as traditional gospel and contemporary music for worship and praise.

Music that has not been designated as religious in nature is said to be secular, and its purposes vary. It is a personal and cultural vehicle for expression and for entertainment. Most popular music is easily recognized as secular. Additionally, many thousands of examples of secular music exist within the genre of art music.

Secular music is defined as being music that is not intended for religious use. Sacred music is defined as being music written for purposes of worship, whether in a religious service or in another setting. This distinction regarding the purpose of the music is the only real difference between secular and sacred music.

Both types of music can be found in many different genres, or forms, of music. Secular music, which may also be known as popular music, can include jazz, country-western, rock, hip-hop, rap, new age, folk, and ethnic varieties of music from other cultures. Church musicians use these same genres to create music for religious purposes.