A satoshi is currently the smallest fraction of a Bitcoin. The unit has been named "satoshi" in collective homage to the founder Bitcoin, Satoshi Nakamoto. Satoshi are to Bitcoin like Pennies are to Dollars - just smaller.
Bitcoin is a form of digital currency, created and held electronically. No one controls it. Bitcoins aren’t printed, like dollars or euros – they’re produced by people, and increasingly businesses, running computers all around the world, using software that solves mathematical problems. It’s the first example of a growing category of money known as cryptocurrency.