Polkadot is a blockchain protocol that connects different blockchains together. Blockchains today have a variety of functions, however they have problems communicating with one another. Polkadot solves this problem through its network of heterogeneous blockchain shards called parachains. As different specialised blockchains are connected, scalability will increase through parallel transactions. All the chains are secured by the Relay Chain. Each individual blockchain may connect with other blockchains through bridges. Polkadot is fork-less through its on-chain governance system.
There are four roles in Polkadot Consensus Mechanism: Validators, Collators, Nominators and Fishermen. Validators secure the Relay Chain by staking DOTs, validating proofs from collators and participating in the consensus with other validators. Collators maintain the shards (blockchains) by collecting shard transactions, and providing proofs for validators. Nominators nominate validators by staking DOTs. Fishermen monitor the network and report malicious activities to validators. Polkadot was originally run on Proof of Authority (PoA) before transitioning to Nominated Proof of Stake (NPoS).
Polkadot provides a framework to build parachains, called Substrate. Any blockchain built on Substrate will work natively with Polkadot, and have all the features such as security and fork-free upgrades. Kusama is a test network for Polkadot. Developers may run their ideas on Kusama and testing on it before deploying into Polkadot.
DOT is the native token in Polkadot. It has three utilities: governance, staking and bonding. All Polkadot holders have the rights in the governance of the Polkadot ecosystem. For securing the Relay Chain and validating the transactions, DOT is used by validators and nominators for staking, and staking rewards. New parachains are added into Polkadot through bonding DOT. Non-active parachains are removed by unbonding DOT. DOT is an inflationary token, thus it does not have any maximum supply.