Polkadot

Polkadot.network is a inter-chain solution to existing blockchain infrastructure. Polkadot unites and secures a growing ecosystem of specialized blockchains called parachains. Apps and services on Polkadot can securely communicate across chains, forming the basis for a truly interoperable decentralized web.

Summary

Polkadot is a network protocol that allows arbitrary data—not just tokens—to be transferred across blockchains.

This means Polkadot is a true multi-chain application environment where things like cross-chain registries and cross-chain computation are possible.

Polkadot can transfer this data across public, open, permissionless blockchains as well as private, permissioned blockchains.

This makes it possible to build applications that get permissioned data from a private blockchain and use it on a public blockchain. For instance, a school's private, permissioned academic records chain could send a proof to a degree-verification smart contract on a public chain.