Polkadot is a sharded heterogeneous multi-chain architecture which enables external networks as well as customised layer one "parachains" to communicate, creating an interconnected internet of blockchains. The cryptocurrency uses proof of stake. The protocol was created by the Ethereum co-founder Gavin Wood and is developed by Parity Technologies. https://en.wikipedia.org/wiki/Polkadot_(cryptocurrency)
Polkadot (cryptocurrency) - Wiki
Polkadot is a free and open source project referring both to an open network protocol and the public network that runs this protocol
Polkadot claims current blockchain protocol implementations inadequately address differences in the needs of disparate users preferences for tradeoffs between privacy, security, and scalability. Additionally it is difficulty for most blockchains to communicate information to each other without an interoperability layer.
The polkadot pre-sale will distribute 50% of the DOT tokens at the genesis block through a Dutch auction. No specific date has been announced, however it will not be available to residents of the USA or China. The sale will use Parity PICOP's, a technology for confirming identity to participate in ICO's, for know your customers (KYC) services. DOT tokens will not exist until release of the polkadot blockchain, estimated for Q3 2019.
The polkadot chain is at the center of the polkdadot architecture providing the relay chain between a variety of parachains and virtual parachains (e.g. Ethereum, Zcash, etc).
The polkadot chain is governed by a proof of stake mechanism with a four varieties of user roles including validators, nominators, collators, and fisherman.