The most critical parts of the Polkadot network are Relay Chain, Parachains, Parathreads, and Bridges.
Most sharded blockchain protocols require a large number of validators on each shard. Polkadot’s erasure coding feature, and the extra checks that happen next, is the edge that allows it to provide the same validity guarantees on each shard with as low as ten validators per shard, ensuring far greater scalability.
Parachains can interact without trust bounds much like smart contracts on Ethereum can interact without trust bounds — they share state and validation logic with the greater network.
Because Polkadot provides the security and validity guarantees, parachains are not subject to normal blockchain attack scenarios, like a 51% attack. Polkadot validators will reject invalid blocks, so a parachain only needs a single honest collator to submit blocks.
This model opens up a new area of crypto-economics that could include token-less parachains, proof-of-stake parachains that use either a native parachain token or dot tokens, or other collator selection mechanisms. Block author information is normally part of the block header’s digest and is easy to verify.
The Cross-Chain Messaging Protocol (XCMP) allows parachains to send messages of any type to each other. The shared security and validation logic of the Relay Chain provide the environment for trust-free message passing that opens up true interoperability.