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.
Sacred music is music that is written for or performed for religious purposes. Dating back to Medieval times, the Catholic church commissioned composers to write music for various services and events related to the church. Today sacred music is still used in church services. Sacred music is also produced and sold for personal purchase and enjoyment.
The essence of secular music boils down to any music that is not sacred. Music that is about a wide variety of topics such as love, loss, hope, joy, and every day events (without explicitly referencing religious concepts) is considered secular.
Some music is considered to be of the "crossover" category if it doesn't focus on God or religion but still hints about spiritual themes. You might hear a crossover song on either religious or secular radio stations.