Blockchains are public, digital, and decentralized ledgers or cryptocurrency transactions. The original cryptocurrency blockchain was Bitcoin but there are over 700 new currencies on the market such as Ethereum, Ripple, and Dash. The power of a blockchain is that there is no single entity controlling the state of transactions. It also forces redundancy in the system by ensuring everyone using a blockchain also maintains a copy of the ledger. Assuming there is no inherent trust in blockchain participants, the system must live in consensus. 

A good question to ask is if we have solved identity management and security with asymmetric cryptography and key exchanges, why are blockchains needed to exchange data or currency?  This is not enough for the exchange of money or data of value. One thing to note is that since the inception of information theory, when two devices communicate, Bob and Alice, they send a message or a bit of data. That information is still retained by Bob even if Alice receives a copy. When exchanging money or contracts, that data must leave one source and arrive at another. There is only a single instance. Authenticity and encryption are tools needed for communication but a new method to transfer ownership had to be invented:

Blockchain secure cryptocurrencies are particularly relevant to IoT. Some example use cases include:

• Machine to machine payments: The IoT needs to ready itself for machine exchanging services for currency. 
• Supply chain management: In this case, the movement and logistics in managing inventory, moving goods, and logistics can replace paper-based tracking with blockchain immutability and security. Every container, movement, location, and state can be tracked, verified, and certified. Attempts to forge, delete, or modify tracking information becomes impossible. 
• Solar energy: Imagine residential solar as a service. In this case, solar panels installed on a customer home are generating energy for the home. Alternatively, they can also send energy back to the grid to power someone else (perhaps in exchange for carbon credits).