Polkadot's interoperability capabilities allow dApps to be automatically transferred between different blockchains, for example if their community chooses a new chain infrastructure or wishes to upgrade their existing one. If they should desire it, all chains will still have access to a shared decentralized virtual machine capable of running high-level computation and hosting user-generated smart contracts.
This new architecture will allow for a public chain which is fully compatible with the Ethereum ecosystem allowing existing dApps to operate on Polkadot without modification, but at the same time gives these projects access to blockchain features such as interchain atomic swaps and faster block times when they need them. Additionally it allows independent chains to evolve their own governance model away from pure proof of work or proof of stake models in the case of public networks and towards more hybrid structures that may be better suited to private chain needs, such as delegated proof of authority. The common security model would provide protection against replay attacks between chains while retaining individual autonomy over consensus rules and chain assets.
Seeing how different blockchains can be in terms of vision, technical implementations and even governance models, Polkadot aims to become a generic multi-chain infrastructure which will allow blockchain-to-blockchain communication in a trustless manner.
While it is currently envisioned that most chains will use proof of work or proof of stake as their consensus algorithm, this may change in the future when different teams build P2P networks better suited for specific use cases. While some degree of interoperability can take place using bridges between blockchains, Polkadot provides developers with the ability to create new parachains utilizing any architecture they desire while being able to communicate with other chains directly.
Polkadot is composed by several independent P2P networks that choose consensus mechanism that fits best to its needs, although proof of work seems to be the most common.
This technology is new and under development so it's highly recommended not to use it on mainnet until all security checks are done.
Polkadot is currently under development by the Parity Technologies team led by Dr Gavin Wood. While they are keeping most of the protocol details secret for now, implementers are waiting with excitement for its release scheduled on 2019.
Node Network: A node network can be seen as a blockchain that runs an independent Polkadot client.
The first implementation of the Polkadot client was done by Parity Technologies, but it's planned to be ported on several other languages like Rust or Python in order to make it easier for developers.
Parity nodes are already running on the mainnet and can be controlled using WebSockets API. You can connect to a running node using wss://parity-poc-v3.herokuapp.com/ws.
Testnet: The Polkadot testnet is currently available for everyone to use and will be used during the development process to run tests, deploy parachains or integrate new features in the protocol. It uses Proof of Authority (PoA) for consensus and allows developers to run their own nodes.
