Bitcoin is a decentralized digital currency, without a central bank or single administrator, that can be sent from user to user on the peer-to-peer bitcoin network without the need for intermediaries. Transactions are verified by network nodes through cryptography and recorded in a public distributed ledger called a blockchain.

Bitcoin is unique in that there are a finite number of them: 21 million.

Bitcoins are created as a reward for a process known as mining. They can be exchanged for other currencies, products, and services.

As of February 2015, over 100,000 merchants and vendors accepted bitcoin as payment.

Bitcoin is different from fiat currency in several ways:

1. Decentralization: Bitcoin is decentralized and not subject to government or financial institution control.

Fiat currency, on the other hand, is subject to central bank control.

2. Limited supply: There will only ever be 21 million bitcoins in existence.

This makes Bitcoin more like gold than fiat currency, which can be printed by governments at will.

3. Pseudonymity: Bitcoin transactions are pseudonymous, meaning that users’ identities are not attached to their transactions.

Fiat currency transactions are not anonymous.

4. Security: Bitcoin transactions are secured by cryptography and each transaction is verified by the network before it is recorded in the blockchain.

Fiat currency transactions are not secured by cryptography and are not verified by the network before they are recorded.

5. Immutability: Once a Bitcoin transaction is recorded in the blockchain it cannot be altered or reversed.

Fiat currency transactions can be reversed or altered by the issuing authority (e.g., a bank).

In conclusion, Bitcoin is different from fiat currency in several key ways that make it attractive as an investment and payment system: decentralization, limited supply, pseudonymity, security, and immutability.