At its core, the habit is a visible sign of the nun's complete consecration to God and unity with other nuns in a religious community. As different orders of nuns have different habits, each order's habit serves to distinguish the community from other orders and highlight characteristics of each order.
As the religious life is, in a sense, a renunciation of the good things of the world for the greater good of heaven, the distinctive garb of a nun highlights this new way of life. The habit also shows the public nature of the nun's commitment to God, and those habits that cover the entire body symbolize the consecration of the entire person to both God and humanity. Nuns in some orders wear their habits all of the time, which symbolizes that their way of life is not merely a day job or temporary thing.
Distinctions among the habits of different orders of nuns or sisters highlight important aspects of each order. Carmelites wear brown to represent both the virtue of humility and the wood of the cross upon which Jesus was crucified. The Sisters Adorers of the Royal Heart of Jesus wear blue in honor of Mary, the mother of Jesus.