According to Christian doctrine there are three persons in the one God: the Father, the Son (Christ) and the Holy Spirit (sometimes called the Holy Ghost). The textual basis of this Trinity is found in the New Testament (Matthew 28: 19).
When the Holy Spirit is depicted in paintings it is represented by a white dove, a symbol of purity and the soul. For example, see the dove which hovers above Christ in Piero della Francesca's 'Baptism of Christ', and that which descends to the Virgin Mary in Duccio's 'Annunciation'.