Cosmos is a network connecting many independent distributed ledgers (e.g., Ethereum, Bitcoin) to achieve interoperability across blockchains. Its goal is to "create an Internet of Blockchains", i.e., "a network of blockchains where each participant is able to communicate with each other in a decentralized way".
Cosmos is built on a set of independent blockchains referred to as zones, which are powered by the Tendermint Core consensus mechanism - a Byzantine Fault Tolerance (BFT) consensus mechanism used to scale public Proof-of-Stake (PoS) blockchains.
Defined as a "blockchain 3.0", Cosmos relies on three additional core elements:
Cosmos Hub: the first of the "interconnected blockchains" that is referred to as the Cosmos Network.
IBC Protocol: the Inter-Blockchain Communication protocol is a standardized interoperability protocol attempting to connect the global economy with blockchain technology.
Cosmos SDK: it allows developers to build their custom blockchain and to connect it through the ITB protocol. For instance, Terra and Binance Chain are built with the Cosmos SDK.
ATOM is the native asset of Cosmos and is used for on-chain governance. Specifically, ATOMs can be staked by zone validators to join hubs and conversely, their locked tokens would be slashed in case of malicious behavior (i.e., bonding mechanism). In addition, Cosmos Hub has a similar gas system as Ethereum, and requires ATOM to be used for transaction fees.