Bitcoin (BTC) is the first decentralized cryptocurrency with the most famous, popular, notable and highest market capitalization.
Bitcoin is pseudonymous, meaning that funds are not tied to real-world entities but rather bitcoin addresses. Transacting parties do not need to know each other's identity in the same way that a store owner does not know a cash-paying customer's name. Owners of bitcoin addresses are not explicitly identified, but all transactions on the blockchain are public.
Balances are kept using public and private keys — long strings of numbers and letters created by some specific mathematical encryption algorithm. The public key (comparable to a bank account number) serves as the address which is published to the world and to which others may send bitcoins. The private key (comparable to an ATM PIN) is meant to be a guarded secret, and only used to authorize Bitcoin transmissions.
There are only 21 million Bitcoins that can be mined in total.