Bitcoin (expressed as BTC or ฿) is a cryptocurrency whose value is generally linked to US dollars.
Just like normal currency the exchange rate varies.
You can find today's value by searching 'BTC to XXX' on a search engine, where XXX equals your currency,
e.g. 'BTC to USD' - 'BTC to GBP' - 'BTC to EUR'
You can transact in Bitcoins or use exchanges to convert Bitcoins into your currency.
A satoshi is one hundred millionth of a Bitcoin (0.00000001 BTC or ฿0.00000001)