The best place to start exploring Celo depends on your interests and objectives:
Alfajores, the Developer Testnet: If you're a developer considering building on the Celo platform, or want to try out the Celo Wallet, start with Alfajores. You can obtain a balance immediately and use it for experimenting with. The SDK tutorials assume you are using Alfajores. The network is managed by cLabs, who operate all of its validators, and its software and deployed contracts will track mainnet.
Baklava, the Node Operator Testnet: If you're interested in operating one or more nodes that make up Celo's infrastructure, start with Baklava. Baklava's validators and full nodes are operated by the community. It is intended to provide a venue to test new builds of the Celo Blockchain software, trial on-chain governance proposals, and for node operators to build operational best practices. Support for developers and users of the Wallet is not the primary focus.
Mainnet: The production Celo network, Mainnet was previously known as the Release Candidate 1 network. Deployed by the Celo community starting 4/22/20, the network is currently working towards enabling the Celo stability mechansim. Much of the activity is being driven by the validator organisations that featured on the leaderboard of The Great Celo Stake Off.