The core NEAR blockchain runs using open source code and there are many instances of it running. Each is a completely separate network with separate validators and separate state. For the purposes of application development, there are only a few networks that you will likely find relevant:
MainNet is the network where user-facing or value-holding applications should be deployed. This is intended for apps that are production ready, and have already gone through thorough testing and, if necessary, independent security reviews.
NEAR's MainNet is currently in undergoing a multi-phase rollout. Restrictions are placed on account creation and token transfer until the final stage. Find more info on this approach to releasing MainNet here: The Road to MainNet and Beyond.
MainNet status is available at https://rpc.testnet.near.org/status
The MainNet explorer is available at https://explorer.mainnet.near.org/
The flag for this in near-cli is 'production'
MainNet is the only network where state is guaranteed to persist over time (subject, of course, to the typical security guarantees of the network's validation process).
TestNet is intended for development of apps on the NEAR platform before they are released to MainNet.
TestNet status is available at https://rpc.testnet.near.org/status
The TestNet explorer is available at https://explorer.testnet.near.org/
The flag for this in near-cli is 'development'
We make every attempt to maintain the integrity of network state across updates, but this is still a volatile network that receives heavy testing. It is therefore recommended that all user-facing applications are deployed to MainNet.
BetaNet is intended primarily for validators who wish to test their infrastructure.
Releases happen weekly.
BetaNet status is available at https://rpc.betanet.near.org/status
The BetaNet explorer is available at https://explorer.betanet.near.org/
The flag for this in near-cli is 'betanet'
State is maintained as well as possible but with no guarantees and high volatility.
LocalNet is intended for developers who want to work with the NEAR platform independent of the public blockchain. You will need to generate nodes youerself.
More on local development here
The flag for this in near-cli is 'local'
Staging/testing environments are mostly for us to test our code and run CI. We don't recommend you use these unless you're contributing to the code base.
The flags in near-cli are 'test', 'ci' and 'ci-betnet'.