[[ch00_intro_what_is_bitcoin]]
== Introduction

=== What is Bitcoin?

Bitcoin is digital money, a currency for and of the Internet. It is not owned or controlled by any company, group or country. It exists as a set of standards and reference software and as a running network with several thousand nodes worldwide. Behind the scenes, bitcoin is a network, a protocol, a standard and a currency. For now think of it simply as digital money that can be sent, received and stored by anyone, worldwide simply by downloading compatible software and joining a network. 


Under the hood, bitcoin is the culmination of decades of research in cryptography and distributed systems and represents four key innovations brought together in a unique and powerful combination. Bitcoin consists of a de-centralized peer-to-peer network, a public transaction ledger, a de-centralized mathematical and deterministic currency issuance, a de-centralized transaction verification system and a set of powerful APIs. All of these are "bitcoin", and each of these aspects of bitcoin will be examined in this book.

More than all of these parts, bitcoin is a digital economy platform, just like the Internet is a digital communications platform. With bitcoin, it is possible to build entire new financial systems, transaction types and economies on top of a purely digital, instantaneous and frictionless platform, an Internet for money.

=== Stories

It is easiest to experience bitcoin from the perspective of a few specific stories that we will explore in detail throughout the book. 

Each story represents a specific real use of bitcoin in different contexts.


==== Alice buys a cup of coffee from Bob's Cafe

Alice wants to buy a cup of coffee using bitcoin. She visits Bob's Cafe, a coffee shop that accepts bitcoin payments, as advertised by a sign declaring _"Bitcoin Accepted Here"_ in the window. At the counter, the prices may be listed in a local currency like Euros or Dollars. At the register, Bob would ring up a coffee, displaying 

----
Total:
$1.50 USD
0.015 BTC
----


Or Bob might say _"That's one-dollar-fifty, or fifteen milibits"_.

Alice would use a smartphone to scan the barcode on display and send the payment. Her smartphone would show a payment of +0.0150 BTC+ to +Bob's Cafe+ and she would select +Send+ to authorize the payment. Within a few seconds (about the same time as a credit card authorization), Bob would see the transaction on the register, completing the transaction. Alice has purchased a cup of coffee for 15 millibits (or 0.015 bitcoin)

==== A currency

Bitcoin is a currency, the operates much like any "foreign" currency. The main difference is that it is not issued by a national government. Bitcoin currency units are called "bitcoins". Unlike traditional currencies, bitcoins are divisible to much smaller units. The smallest unit is the _satoshi_, one hundred-millionth of a bitcoin (1/100,000,000). Bitcoin can be exchanged for other currencies at specialized currency exchanges that support crypto-currencies like bitcoin. There, a customer can exchange US dollars ($) or Euros (E) for bitcoin, at the prevailing market exchange rate.



==== A network and protocol

Bitcoin operates on top of a peer-to-peer network, also called "bitcoin". The bitcoin network is used to propagate transactions, new blocks and alert messages. The network operates using a relatively simple network protocol for peer discovery and blockchain replication. 


[[table_units]]
table of bitcoin units from bitcoin wiki

One interesting feature of bitcoin is that the issuance of the currency decreases automatically over time, halving every four years, reaching an absolute maximum of 21 million bitcoins issued sometime around the year 2140. 

[[chart_bitcoin_decreasing_issuance]]
Chart of decreasing issuance over time

==== Transactions

People can pay for goods and services using bitcoin as the currency. 

Bitcoin transactions, which transfer value from one bitcoin address to another, are recorded in a distributed ledger, called the _blockchain_. In simple terms, think of the ledger as a book with lines like this:

----
A gives 1 bitcoin to B
C gives 2 bitcoin to D
----

The ledger is a record of all bitcoin transactions and can be independently verified by every node.

==== A distributed ledger

Bitcoin's core innovation is, somewhat ironically, completely de-centralized. It is the _blockchain_, a distributed, timestamped ledger. The ledger consists of a cryptographically verified chain of _blocks_, each of which contains transactions, new coins and a signature (hash) of the previous block. Each full bitcoin node in the network will keep a complete local replica of the blockchain, and independently verify all transactions and balances from that replica. 

==== Proof of work and distributed issuance (mining)


==== A transaction language

==== An API

=== Getting Bitcoin

==== Obtaining the bitcoin software
==== bitcoin-qt, bitcoind
==== mobile client

=== History of bitcoin

==== A brief history of money
==== A brief history of crypto currencies
==== Public key cryptography and crypto-currency
==== Peer-to-Peer networks
=== The history of bitcoin
==== Why would I use bitcoin
===== As a merchant
===== As a consumer
===== As a developer, integrator
===== As an entrepreneur
===== As an investor