What: Bitcoin spelled with a small (b) references Bitcoin as a currency in any amount sent or received.
Why: All amounts on the Bitcoin blockchain are denominated in “satoshi’s”. The source code also uses “satoshi” when specifying an amount of bitcoin.
Examples: in other units >
BTC: One Bitcoin
mBTC: Milli Bitcoin, 1 thousandth of a BTC (0.001)
uBTC: Micro Bitcoin, 1 millionth of a BTC (0.000001)
Satoshi: smallest possible Bitcoin unit (0.00000001)
References:
https://en.bitcoin.it/wiki/Units
https://en.bitcoin.it/wiki/Satoshi_(unit)