Bitcoin is the name of the protocol (in fact a protocol that defines the rules of a technological and at the same time economic system) released by Satoshi Nakamoto at the beginning of 2009.

The first application is bitcoin, the first decentralized trustless payment system.

Here you can get the seminal white paper.

Later on, came other crypto-currencies such as litecoin (using Scrypt instead of SHA256 as a Proof of Work algorithm), peercoin (the first Proof of Stake crypto-currency) and primecoin, the first non Hashcash crypto-currency.

However, those crypto-currencies do not have a stable price, making them useless as currencies you would use in daily use. This is why, NuBits, the first crypto-currency to be stable in price was released in September 2014.