Cosmos (ATOM) is a blockchain ecosystem focused on interoperability. It is often described as a way for independent blockchains to communicate using shared standards and infrastructure.
What Cosmos is used for
Developers can build app-specific blockchains instead of putting every application on one chain. Those networks can then connect through interoperability tools when configured to do so.
What ATOM represents
ATOM is the asset associated with the Cosmos Hub, including staking and governance roles. The wider Cosmos ecosystem includes many separate chains, so users should not assume every Cosmos-related project is the same as ATOM.
Cosmos is important because it treats crypto as a network of blockchains rather than one winner-take-all chain.