A collision domain is, as the name suggests, a part of a network where packet collisions can occur. A collision occurs when two devices send a packet at the same time on the shared network segment. The packets collide and both devices must send the packets again, which reduces network efficiency. Collisions will happen in an Ethernet Network when two devices simultaneously try to send data on the Shared Media, since Shared Media is half-duplex and sending and receiving is not supported at same time. Please refer CSMA/CD to learn how Ethernet avoid Collision.
Collisions are a normal part of life in an Ethernet network when Ethernet operates in Half-duplex and under most circumstances should not be considered as a problem.
A Collision Domain is any network segment in which collisions can happen (usually in Ethernet networks). In other words, a Collision Domain consists of all the devices connected using a Shared Media (Bus Topology or using Ethernet Hubs) where a Collision can happen between any device at any time.
Broadcast is a type of communication, where the sending device send a single copy of data and that copy of data will be delivered to every device in the network segment. Brodcast is a required type of communication and we cannot avoid Broadcasts, because many protocols (Example: ARP and DHCP) and applications are dependent on Broadcast to function.A Broadcast Domain consists of all the devices that will receive any broadcast packet originating from any device within the network segment.