A satoshi is the smallest fraction of a bitcoin, to be exact a hundred millionth of a Bitcoin.

1 Satoshi = 0.00000001 ฿
10 Satoshi = 0.00000010 ฿
100 Satoshi = 0.00000100 ฿ = 1 Bit / μBTC (you-bit)
1,000 Satoshi = 0.00001000 ฿
10,000 Satoshi = 0.00010000 ฿
100,000 Satoshi = 0.00100000 ฿ = 1 mBTC (em-bit)
1,000,000 Satoshi = 0.01000000 ฿ = 1 cBTC (bitcent)
10,000,000 Satoshi = 0.10000000 ฿
100,000,000 Satoshi = 1.00000000 ฿