Polkadot is a platform that allows diverse blockchains to transfer messages, including value, in a trust-free fashion; sharing their unique features while pooling their security. In brief, Polkadot is a scalable heterogeneous multi-chain technology.
Polkadot is heterogeneous because it is entirely flexible and makes no assumption about the nature or structure of the chains in the network. Even non-blockchain systems or data structures can become parachains if they fulfill a set of criteria.
Polkadot may be considered equivalent to a set of independent chains (e.g. a set containing Ethereum, Ethereum Classic, Namecoin and Bitcoin) except with important additions: pooled security and trust-free interchain transactability.
For more information, please see the Polkadot wiki here: https://wiki.polkadot.network