Mainnet and testnet. Of course, you know that, right? They have a few differences.

Mainnet is like the real deal. It's where all the actual transactions happen. Imagine it as the main road where people drive their cars. It's where all the real money and stuff gets moved around. If you're using Bitcoin or Ethereum, that's what you're using.

Testnet is like a practice road. It's where developers test out new ideas and make sure everything works before they put it on the main road. They use fake money and it's totally safe. If something goes wrong, it doesn't affect the real money on the main road.

So, mainnet is for real transactions, and testnet is for testing and making sure everything works right before it goes live.