The Ethereum Main Network is the key component in which all the technologies meet for creating a platform in which you can deploy smart contracts and run decentralized applications. Although it's the main network, some developers prefer to leave their dApps running on testnets to avoid expensive costs. In any case, it's better to deploy your final application in the Mainnet, where more users will be able to use your decentralized applications with real Ether and the latest updates.
Mainnet is also known as Homestead, and it was released in 2015 with the creation of Ethereum as the production release where all the real-world use cases are deployed. It features a 15-second block generation that varies depending on miners, and it runs PoW, which depends on having a strong network of nodes sustaining the network with their mining efforts, otherwise...