Bitcoin (BTC) is a cryptocurrency created in 2009 by an anonymous person named Satoshi Nakamoto. Bitcoin was designed by this mysterious character as electronic payment based on cryptography. It was originally released as open source software and is widely considered the first decentralized cryptocurrency.
Unlike FlAT (EUR, PLN, GBP), Bitcoin is not controlled by any central authority - thanks to which it is independent of governments and financial institutions. To make transactions more secure, Bitcoin uses special data-base system, called blockchain. Bitcoin is also limited supply - you can't "create" more of this cryptocurrency than was initially assumed. This limit for BTC is 21 million.
Bitcoin is a deflationary currency, which means that its value increases over time. Thanks to this, currency inflation is practically impossible. It can be stored in dedicated wallets (private or stock market), invested or paid for purchases.
Ethereum is called "Bitcoin little brother".