The NXT Testnet is a copy of the NXT system where you can try out every functionality.
You should not use your real NXT passphrase or account while using the Testnet!
How to access the NXT testnet:
1) Download the latest NXT Version from the forum.
2) Unzip the version and set Testnet to 'true' in:
nxt/conf/nxt-default.properties - nxt.isTestnet=true
3) Run localhost:6876 in your browser and log in with your Testnet Account.
You can receive your first Testnet if you write a short comment here:
Most of the time you will receive 5k TestNXT to test. If you need more you should address abctc directly.