What is the significance of priest breaking the bread (Host) in a Catholic mass? In a Catholic mass, just before the distribution of communion to people, the priest breaks the Host in to pieces and people simultaneously acclaim: Lamb of God you take away the sins of the world, Have mercy on us.......

What is the significance of this fraction of bread (Host)?  Does it have any parallel to the act of sacrifice for atonement of sins:  like sacrificing of lambs in OT and then sacrifice of Jesus himself for our sins?   

Also the shape of this fractioned Host is different at different times. Are there any laid down norms for this?
The meaning of the fraction of bread is, that Christ is 'distributing himself' to the Faithful. As the prayer in Liturgy of St. John Chrysostom states


  The Lamb of God is broken and distributed; broken but not divided. He is forever eaten yet is never consumed, but He sanctifies those who partake of Him.


Also, dropping the part of the Host into the Chalice is a sign of Resurrection - the reunion of Blood and Body, of Body and Soul.

There is no specific meaning to the shape of breaking in Catholic churches. The priests typically break the host roughly in half and then break a small particle off one of the halves. (In Orthodox churches and Eastern Catholic tradition, the host is a cube, a quarter of which into the chalice).