The TestNet is separate from the Bitcoin network, used primarily by developers for testing Bitcoin-related applications. Although similar, the TestNet and the Bitcoin network are not compatible, meaning you can't use bitcoins on the TestNet, and you can't use TestNet coins on the Bitcoin network. The two networks behave in much the same way however, with a few key differences:
Previous attempts to add value to TestNet coins have resulted in the network being reset. There is no point to buying or selling TestNet coins for this reason.
There have been 3 generations of TestNet thus far, with each one effectively resetting the money supply. The latest generation also addressed the issue with mining difficulty.
The third generation of TestNet (TestNet3) added a new rule for mining coins. If nothing is mined for 20 minutes, the difficulty will reset to the minimum (1.0) for the next block. This allows more blocks to be mined and keeps the network accessible to more miners.
Although TestNet coins have no value, like Bitcoin, there is a finite supply, and you must acquire them somehow. Some of the more popular ways are as follows:
TestNet coins can be mined using the same software used to mine bitcoins. A web search for "testnet mining" should turn up the information necessary to connect your mining hardware to a TestNet mining pool, and begin acquiring coins. If that sounds like too much work however, there are easier ways.
One simple way to acquire TestNet coins is through a web-based service known as a faucet. These are services set up by individuals or organizations with TestNet coins to spare. A web search for "bitcoin testnet faucet" should point you to the latest available faucets.