The chasuble is worn over the Alb and stole.  “The vestment proper to the priest celebrant  at Mass and other sacred actions directly connected with Mass, unless otherwise indicated, is the chasuble, worn over the alb and the stole” (GIRM no. 337).  Bishops and priests wear the chasuble, the wide circular robe covering the whole person.  According to the meaning that goes back to Isidore of Seville the term chasuble means "small house". It was used to define the cell inhabited by the monk and the small country chapel. So, this explanation fits the typical shape of the garment that completely envelops the wearer.

The chasuble is worn by the newly ordained priest with the meaning of being clothed with Christ and clothed with the people of God entrusted to him. 

Like the stole, the chasuble can be in different  shape or style and follow liturgical colours.  

(For a particular insight into the history of the chasuble, see the special insert in «La Vita in Cristo e nella Chiesa», 3/2011, pp 25-44).