Relay Chain offers sharded security for blockchains that run parallel (Parachains) in the platform. The transactions executed through Parachains are validated through a system called Relay chain.
Parachains is considered as an application-specific data structure that validates the validators of the Relay Chain. It has the capability to support parallel transactions that handle the scalability of the system and connect the Relay Chain.
Parathreads possess the feature of passing messages between other Parthreads or Parachains through XCMP, and Polkadot’s validators secure it. It has a fixed registration fee that costs less than acquiring the Parachain slot.
Bridges contain special blockchains that connect the external networks via shards for distributing the data for parallel processing to increase the throughput rate and reduce the processing time.
See more - https://carawilliam-99.medium.....com/the-world-of-pol