Features of Polkadot
Bitcoin and Ethereumare proof-of-work (PoW) blockchains where security relies on assumptions about processing power; alternative proof-of-stake (PoS) systems' security instead rely on incentives and the ability to destroy security deposits. These differences present difficulties for one blockchain to trust another. Another challenge that blockchain technologies need to tackle is scalability. Existing blockchain systems generally have high latency and can only have tens of transactions per second, whereas credit card companies such as Mastercard or Visa carry out thousands of transactions per second.
Efforts are in place to upgrade Ethereum to Proof of Stake.
Sharding Explained
One prominent solution to scalability for blockchains is to run many chains in parallel, often called sharding. Polkadot is a multi-chain system that aims to gather the security power of all these chains together in a shared security system.