What is a subnet?
A subnet is a special kind of blockchain within the Internet Computer network that can seamlessly integrate with other blockchains to increase capacity. The Network Nervous System combines nodes from independent data centers to create subnets, which are used to host canisters, an evolution of smart contracts. Each subnet runs its own blockchain, and canisters run on-chain in such a way that they can transparently call canisters on other subnets/chains. In fact, there is no difference between calling a shared canister function on the same subnet or a different subnet, it's just a function call within a seamless universe of secure code.
The subnets are transparent to canister code and users of canisters — developers and users just interact with the Internet Computer, while the ICP protocol securely and transparently replicates data and computation across subnet nodes in the background. The system is more secure than a traditional blockchain because the precise decentralization of data and computation is controlled by the protocol rather than left to chance. Pooling (traditional PoW, PoS) or the creation of validator nodes with vast amounts of stake that create more blocks (as per PoS), is not possible. Direct interaction between subnets, and with subnets, is enabled by "Chain Keys," which are made possible by novel advanced cryptography developed by DFINITY.