In order to use the IOTA tangle you have to interact with the nodes that make up the IOTA network. There is the main IOTA network (Mainnet) that hosts the IOTA token and is used for production applications.
However if you want to test the functionality of your application without using real tokens you'll want to use the Public Testnet.
The IOTA Foundation has setup the testnet to provide a comparable network environment for developers to work in while creating their new IOTA based apps.
While we have created a similar environment, the testnet differs in the following ways:
This testnet is only accessible from Foundation run IRI nodes. The endpoint is load balanced to ensure stability when testing application. We are looking to provide satellite nodes for automatic peering in the future.
Minimum Weight Magnitude is 9
The Testnet requires a minimum of 9 MWM (Minimum Wieght Magnitude). This vastly reduces the difficulty of the Proof of Work (PoW) required to send a transaction. Given this you will see your application or solution operating slower on the Mainnet when it is in production.
On the testnet, there is a service is available for use to reduce the PoW load on your simulation or application while they do transactions. This code is open source so you will be able to use the service privately for your mainnet applications.
We also have an faucet which dispenses Testnet tokens. This enables you to test your application with tokens that have 0 value.
The Testnet the IOTA Foundation is functionally the same in terms of structure as the public.
Given this you can expect your applications to work immediately with the Mainnet when you applications are ready.
Mutually Peered Full Nodes
The Testnet has about 8 nodes operated by the Foundation, these are are peered to each other. These nodes are functionally the same as a full node on the IOTA Mainnet apart from their reduced requirement for Minimum Magnitude Weight (MWM) of 9.
In order to interact with these node's IRI API you will access it through a load balanced endpoint.
There is a primitive Coordinator running on the Testnet to protect against attacks. They operate in the same fashion, issuing transactions referencing transaction on the network which are not malicious.
 - The Mainnet has no means of being mapped, so this is an assumption.