The Catholic Church provides three different forms of celebrating the Rite of Marriage. When two Catholics are marrying, the celebration will normally take place within a Mass. The second form, which does not include a Mass, is used when a Catholic marries another baptized Christian. A third form, also outside Mass, is usually celebrated when a Catholic marries someone who is not baptized. The second and third forms are structured around the celebration of the Liturgy of the Word.
The couple chooses one of these options based on their particular circumstances in conversation with the priest or deacon who will witness the marriage vows. When a deacon leads the liturgy, which is increasingly common, the wedding is celebrated outside of Mass even when two Catholics marry.
This video explains more about the Rite of Marriage and what to expect at a Catholic wedding.