Bitcoin(BTC) is a cryptocurrency(digital), internet money, based on peer 2 peer system, completely decentralized, independent from goverments, open-source system. It was invented by Satoshi Nakamoto in 2009. Since that day, it became the most popular digital currency in the world.
It allows you to do worldwide transactions, via your computer, smartphone or tablet, with small fees. It can be transfered to anyone with bitcoin address. And you can even spend it in some online shops. There ale also some rumors about plans to create bitcoin card that will allow you to pay with it everyday just like you do payments with your normal debit card. Right now cost of BTC is rapidly rising what can encourage to investments in BTC.

Bitcoins have also dark side. Because they are completely free and transactions can become untraceable, they are used as unoffical currency of the Darknet. Accessible via TOR, where you can use BTC to pay for illegal drugs, hitmans, etc. But they shouldn’t be considered as an evil tool. We must remeber that you can use a knife to hurt somebody, but if you don’t want to, you won’t do it.

For more informations about bitcoins visit: about Bitcoin
Bitcoin wiki
Official Bitcoin site