Polkadot is a heterogeneous multichain withshared security and interoperability.
Relay Chain
The Relay Chain is the central chain ofPolkadot. All validators ofPolkadot are staked on the Relay Chain in DOTand validate for the Relay Chain. The Relay Chain is composed of a relatively small number oftransaction types that include ways to interact with the governance mechanism, parachain auctions,and participating in NPoS. The Relay Chain has deliberately minimal functionality - for instance,smart contracts are not supported. The main responsibility is to coordinate the system as a whole,including parachains. Other specific work is delegated to the parachains, which have differentimplementations and features.
Parachain Slots
Polkadot can support a number of executionslots. These slots are like cores on a computer's processor (a modern laptop's processor may haveeight cores, for example). Each one of these cores can run one process at a time.Polkadot allows these slots using twosubscription models: parachains and on-demand parachains. Parachains have a dedicated slot (core)for their chain and are like a process that runs constantly. On-demand parachains share slotsamongst a group, and are thus more like processes that need to be woken up and run less frequently.
Most of the computation that happens across thePolkadot network as a whole will be delegatedto specific parachain implementations that handle various use cases.Polkadot places no constraints over whatparachains can do besides that they must be able to generate a proof that can be validated by thevalidators assigned to the parachain. This proof verifies the state transition of the parachain.Some parachains may be specific to a particular application, others may focus on specific featureslike smart contracts, privacy, or scalability — still, others might be experimentalarchitectures that are not necessarily blockchain in nature.
Polkadot provides many ways to secure a slotfor a parachain for a particular length of time. On-demand parachains share slots with otherparachains. Both regular and on-demand parachains have the same API; their difference is economic.Parachains will have to reserve DOT for theduration of their slot lease; on-demand parachains will pay on a per-block basis. Parachains canswitch between being on-demand and permanent.
Parachains connected to the Polkadot RelayChain all share in the security of the Relay Chain.Polkadot has a shared state between the RelayChain and all of the connected parachains. If the Relay Chain must revert for any reason, then allof the parachains would also revert. This is to ensure that the validity of the entire system canpersist and no individual part is corruptible.
The shared state ensures that the trust assumptions when usingPolkadot parachains are only those of theRelay Chain validator set and no other. Since the validator set on the Relay Chain is expected to besecure with a large amount of stake put up to back it, parachains should benefit from this security.
Interoperability
XCM
XCM, short for cross-consensus message, is a format and not a protocol. The format does not assumeanything about the receiver or senders consensus mechanism, it only cares about the format in whichthe messages must be structured in. The XCM format is how parachains will be able to communicatewith one another. Different from XCMP, which is short for cross-chain messaging protocol, XCM iswhat gets delivered, and XCMP is the delivery mechanism. The best way to learn more about XCM is byreading the specification.
Bridges
A blockchain bridge is a connection that allows for arbitrary datato transfer from one network to another. These chains are interoperable through the bridge but canexist as standalone chains with different protocols, rules, and governance models. InPolkadot, bridges connect to the Relay Chainand are secured through the Polkadot consensusmechanism, maintained by collators.
Polkadot uses bridges to bridge the future of Web 3.0, as bridges are fundamental toPolkadot's interoperable architecture byacting as a [secure and robust] communication channel for chains in isolation.
Validators
Validators, if elected to the validator set, produce blocks onthe Relay Chain. They also accept proofs of valid state transition from collators and receivestaking rewards in return.
Validators are required to keep enough parachain blocks available for later use in their localstorage. Those blocks are retrievable by peers who lack that information, so that they can reliablyconfirm the issued validity statements about parachain blocks. The (AnV) protocol consists ofmultiple steps for successfully upholding those responsibilities.
Nominators
Nominators bond their stake to particular validators in order tohelp them get into the active validator set and thus produce blocks for the chain. In return,nominators are generally rewarded with a portion of the staking rewards from that validator.
Collators
Collators are full nodes on both a parachain and the Relay Chain.They collect parachain transactions and produce state transition proofs for the validators on theRelay Chain. They can also send and receive messages from other parachains using XCMP.
Parachain blocks themselves are produced by collators, whereas the relay chain validators onlyverify their validity (and later, their availability).
