Recall that parachains are shards in the relay chain network. Each parachain defines a runtime, or state transition function, specific to their parachain. A relay chain without parachains is not helpful from a user's perspective, as parachains provide application-specific platforms that can work in parallel.
Examples of Existing Parachains
Many existing parachains on Polkadot and Kusama all of which harness the relay chain's validation schemes to facilitate their application-specific parachains. Some are system parachains, which fulfill some specific functionality for the ecosystem. Below are examples of these system parachains:
- Asset Hub - a generic assets parachain that provides functionality for deploying and transferring assets — both Fungible and Non-Fungible Tokens (NFTs).
- Bridge Hub - a parachain that contains the logic needed for bridging to other relay chains or ecosystems.
- Collectives - The Collectives chain hosts on-chain collectives that serve the Polkadot network. These on-chain collectives will play essential roles in the future of network stewardship and decentralized governance.
Cumulus - Parachain Development Kit (PDK)
The chains mentioned above use Cumulus to build their parachains. Cumulus is a parachain development kit that uses Substrate and FRAME to make building parachains a streamlined process for a developer.
More notably, besides providing the necessary tools for building a Substrate runtime, Cumulus provides the means to fulfill the requirements for validating blocks supplied from collators to the relay chain.
Relation to Substrate & FRAME
Although this diagram is simplified, this is essentially the relationship between these tools. For more information, refer to the documentation for Substrate, FRAME, and Cumulus.
For a more detailed diagram, refer to the official Substrate documentation.