Ethereum is a protocol; there can be multiple independent networks conforming to this protocol that do not interact with each other.
The main Ethereum network (mainnet) is the most important such network. However, other important networks exist besides the mainnet. Most of these are public testnets, which are networks used by protocol developers or smart contract developers to test both protocol upgrades as well as potential smart contracts in a production-like environment before deployment to mainnet. Think of this as an analog to production versus testing servers. It’s generally important to test any contract code you write on a testnet before deploying to the mainnet.
Ether on testnets has no real value; therefore, there are no markets for testnet Ether. Since you need ether to actually interact with Ethereum, most people get testnet ether from faucets. Most faucets are webapps where you can input an address which you request ether to be sent to. You can usually find testnet faucets by Googling.
There are multiple Ethereum testnets. Ropsten is a testnet which uses “Proof-of-Work” as its consensus algorithm, a transaction validation process equivalent to that of the mainnet; it is supported by both Geth and Parity. Rinkeby is a testnet which uses a different “Proof-of-Authority” consensus algorithm. There are several more beyond these.