Cardano is one of the top five, layer one blockchains by market capitalization. Its protocols have been thoroughly peer-reviewed and uses formal methods to ensure that its transactions and smart contracts meet the highest security standards.
As a third-generation blockchain, Cardano supports smart contracts which makes it a programmable blockchain. Smart contracts are deterministic software programs that can be trusted to have the same outcome each time they are run. They are used to enable a wide variety of decentralized finance and record-keeping solutions.
Cardano is designed to support vastly more transactions per block than Bitcoin or Ethereum, all while ensuring very low, predictable transaction costs. It has a layer-2 scalability solution called Hydra that allows it to reach millions of transactions per second. Cardano separates its cryptocurrency settlement layer from the smart contract execution layer. In contrast, Ethereum handles both its ETH crytpocurrrency transactions and smart contracts on the same layer, which often leads to heavy network congestion and very high gas (transaction) fees.
Cardano is the first blockchain to implement a fully-decentralized proof-of-stake consensus protocol with over 3000 independent stake pools validating blocks. These stake pools issue stake rewards every five days in Cardano's native ADA cryptocurrency. The staked ADA remains in owners' wallets and can be withdrawn at any time, unlike other staking or yield farming solutions that have strict vesting requirements.
Thanks to its proof-of-stake architecture, Cardano is a truly "green" blockchain that avoids the significant environmental damage that competing blockchains like Bitcoin and Ethereum are doing by wasting vast amounts of electricity on proofs-of-work, most of which comes from burning fossil fuels.
As some observers like Coindesk have noted "thanks to its advanced technologies, rigorous approach and gift of hindsight, Cardano appears to be in a strong position to challenge Ethereum’s dominance and address some of the most common blockchain problems earlier generation blockchains face, namely scalability, sustainability, interoperability and governance."