Cosmos is a protocol that allows independent blockchains to communicate with each other.
Cosmos can also be considered framework and library of basic software modules. What does that mean? This means that any project can take any suitable module from the library and use it to implement all other project-specific components. This significantly speeds up the development process and increases the reliability of solutions.
Cosmos Hub is the first public blockchain in the Cosmos universe. It acts as a router that facilitates transactions between all the blockchains of the Cosmos ecosystem. Cosmos Hub is also the first public blockchain that has proven in practice that PoS can withstand Sybilla attacks in public blockchains without the need to use external sources.
Cosmos SDK is a platform on which it is easy for developers to build and run sidechains.
Sidechains are a technology that allows digital assets of one blockchain to be used safely in another. In Cosmos, these sidechains are called zones. They can be connected to the Cosmos Hub, and as soon as your sidechain is connected to the Cosmos Hub, it will be compatible with all other sidechains that are connected to the Cosmos Hub.
Tendermint is a consensus mechanism (belongs to the BFT consensus family) that can be used as the first "layer" for any PoS blockchain. In practice, this allows developers to deploy blockchains without having to create everything from scratch.
Cosmos Network is a network of blockchains that are connected to each other via Cosmos Hub.
On on this site you can see all the blockchains compatible with the COSMOS ecosystem, including those that are still in the testnet.
The Decimal blockchain is also part of Cosmos and can be compatible with any of the ecosystem's blockchains.