The TestNet is an alternative Bitcoin/Litecoin block chain used for testing.
This allows application developers or bitcoin/litecoin testers to experiment without having to use real bitcoins or litecoin.
To start using bitcoin and litecoin TestNet you will need to switch your wallet to the testnet mode.
testnet=1 //needed to work under the testnet.
You can get free testnet Bitcoins and testnet Litecoins to test at the following sites:
The real BTCMerch and Sandbox have the same payment logic. The only difference between them is that Sandbox payments go through TestNet.