Cardano's goal is to be the most environmentally friendly blockchain platform. It uses a unique proof-of-stake consensus mechanism called Ouroboros, as opposed to the energy-intensive proof-of-work system currently used by Bitcoin and Ethereum. (Ethereum is also transitioning to the proof-of-stake system through the ETH2 update.)
ADA is a token in a distributed computing platform that allows you to run smart contracts, decentralized applications, side chains, multi-party computing and metadata. The name comes from the nineteenth-century mathematician, Ada Lovelace, who is recognized as the first computer programmer.
ADA tokens power the Cardano platform, much like ETH tokens power the Ethereum platform. They are used to pay transaction fees and are set by reviewers (and delegators) who want to help keep the network secure and stable in return for earning rewards.
In the future, ADA will also be used as a token to allow holders to vote on changes and updates to the Cardano platform.