For a user, Bitcoin is pretty much like cash for the Internet.
Bitcoin is a payment network and a new kind of digital money.
Bitcoin is a decentralized digital currency system.
Bitcoin is a consensus network that enables a new payment system and a completely digital money.
Bitcoin is the first decentralized peer-to-peer payment network that is powered by its users with no central authority or middlemen.
Bitcoin is a cryptocurrency created by Satoshi Nakamoto since 2009.
Bitcoin is the most prominent triple entry bookkeeping system in existence.
Bitcoin can only work correctly with a complete consensus among all users.
The Bitcoin network is sharing a public ledger called the block chain. This data contains every transaction ever processed, allowing a user's computer to verify the validity of each transaction.
The authenticity of each transaction is protected by digital signatures
Bitcoin Official Website
What is Bitcoin?

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Start buying and selling Bitcoin and Ethereum. Buy or sell bitcoins. Get ticker sell or buy price. Request withdrawals. Make deposits.
Coin Base - Currency and Digital Exchange - (API for developers)
Buy Bitcoin Worldwide - Bitcoin Exchanges
Coin Market Cap - Crypto Exchange
Poloniex
Binance
Merchants and Digital Wallets.
BitcoinWide Merchant
Coin Map Merchant
Spend a Bit Merchant
99 Bitcoins Merchant
Bitcoin Merchant tools
Bitrawr Bitcoin
Bitcoin Projects
Bitcoin Vouchers
Bitrefill Buy gift cards for anything under the sun.
Fold
Gyft
Opendime
Learn about bitcoin and cryptocurrencies

Bitcoin Talk Forum and Meetups
Bitcoin StackExchange (Frecuent Questions and Answers)
Bitcoin Conferences And Events
Bitcoin Meetup Groups
Reddit's Bitcoin Community
Bitcoin Groups in Social Networks : Facebook #bitcoin Twitter #bitcoin-market (Live quotes from markets) - Bitcoin Mining (#bitcoin-mining), Transactions Security
Magic Money: The Bitcoin Revolution