A snitch determines which datacenters and racks, nodes belong to. They inform Cassandra about the network topology and allows Cassandra to distribute replicas specifically, the Replication strategy places the replicas based on the information provided by the new snitch.
There are many types of snitches, to name a few:
Dynamic snitching
SimpleSnitch
RackInferringSnitch
Ec2Snitch
PropertyFileSnitch
GossipingPropertyFile
Ec2MultiRegionSnitch
GoogleCloudSnitch
CloudstackSnitch
