ChainLink is the first project of it's kind whose goal is to provide a decentralized oracle service.
Ethereum is currently worth $1,000 because it offers trustless smart-contracts. Smart-contracts are a way to program the money to do things that's guarenteed. An example would be Bob sends Alice 500 RCN instantly when Alice sends Bob 500 REQ.
But there is a major issue with smart-contracts. Smart-contracts can only access blockchain data. In order to access 3rd party (stock prices, gas prices,weather information, etc) they need an oracle. An oracle is a service that converts 3rd party data into smart-contract usable data. An example would be let's say Bob wanted to make a smart-contract bet with Alice on the outcome of the last superbowl. The oracle that provides the game data would pull the ending score information from Yahoo, Google, NFL.com, etc then send the information to Bob's smart-contract.
The problem here is that the oracle is centralizied, and both Bob and Alice would have to trust this single oracle, which could be hacked, to be providing correct data, and because of that there is no reason to be using a guarenteed trustless smart-contract if the data is not trustless.
This is where ChainLink comes in. ChainLink will offer decentralized oracles that provide trustless data. With this system if Bob and Alice wanted to create that smart-contract they could pull information from multiple LINK nodes, and confirm they are all providing the same accurate data. These LINK nodes themselves could then pull each of their information from Yahoo, Google, NFL, etc. Because the nodes are staked with LINK tokens, if one node is providing incorrect data they would risk losing a portion of their staked tokens. LINK nodes providing correct data will be paid in more LINK tokens.
I hope this explanation clears up what ChainLink does and why it has a massive use case.
ChainLink's testnet was just released and it's mainnet is schedule to release later this year.
Edit: If you're interested in staking your own LINK tokens, you should know setting up a node does require some programming knowledge. Linkpool is a site that plans on taking care of all the setup for you, but they will take a 30% cut of your node payouts.
