Blockchain technology will disrupt current business models by making intermediary services obsolete. The term blockchain has become a buzzword worldwide. IT technologists are picking up blockchain books in flocks and starting to read about it in the hope of mastering the basic concepts. People are motivated to become professional blockchain application developers. Unless you pick up a well-written book, it often requires you to read many books and articles before the concepts of the blockchain become clear. To assist you in gaining a concise view of how an end-to-end blockchain application works, we outline a high-level introduction of the basic concepts along with the code-level details explaining how an actual application can be developed step by step. At the end of this book, we cover blockchain use cases in examples to inspire you to work on life-changing projects.

In this chapter, we give an overview of blockchain, along with its key concepts such as cryptography and hash algorithms, the distributed ledger, transactions, blocks, proof of work, mining, and consensus. We cover Bitcoin, the mother of blockchain technology, in detail. We briefly introduce Ethereum by pointing out some limitations of Bitcoin and how they are addressed by Ethereum. While Bitcoin and Ethereum are examples of public blockchains, IBM's Hyperledger is used as an example of enterprise blockchains. At the end of this chapter, we mention the evolution of the blockchain: blockchain 1.0, 2.0, 3.0, and beyond, based on their use cases. Specifically, we will cover the following topics on blockchain:

• A genealogical analogy for blockchain
• The Bitcoin consensus mechanism
• A brief discussion of Hyperledger 
• Blockchain evolution