Cosmos is a decentralised network of independent blockchain powered by BFT consensus algorithm. Cosmos aims to solve common blockchain problems such as scalability, communications, sovereignty and sustainability. Cosmos connects different blockchains together through a number of its features. These are Tendermints BFT, Cosmos SDK and IBC.
Blockchain developments requires three layers: networking, consensus and the application. Tendermint reduces the time for developments by packaging consensus and networking, thus allowing developers to focus their effort on building the applications. As Tendermint does not interfere with the applications, developers can build both public and private blockchains. Tendermint connects with the application through a socket protocol called the Application Blockchain Interface (ABCI). ABCI allows for any programming language to be used, therefore developers does not need to learn new languages.
Cosmos SDK helps the developers in building the application by providing modules for application-specific blockchains. Therefore, developers can easily use these modules for their specific applications, in addition to creating their own modules which would help the SDK modules to expand.
Blockchains are connected together through a system of zones and hubs, and Inter-blockchain Communication Protocol (IBC). Each blockchains built on Cosmos are called zones and are connected to a hub through IBC, or Peg-zones for non-Tendermint chains. The first hub is the Cosmos Hub. Each zones have their own governance, tokens, and applications, and can communicate with other zones through the Hub.
ATOM is the token for Cosmos Hub. Its utility is as staking and staking reward for validating and delegating validators to secure the Cosmos Hub (and the zones connected to it). It is also used for governing the Cosmos Hub. There is no maximum supply of ATOM.