Bitcoin is a cryptocurrency (http://en.wikipedia.org/wiki/Cryptocurrency). It is a completely digital, peer-to-peer, open-source currency that is managed by a global network of computers. Bitcoin is the most prominent cryptocurrency in existence.
Bitcoin can be best understood by reading the developer overview: (http://bitcoin.org/en/how-it-works). The link for the original paper is at the bottom of the page.
What is Bitcoin’s Value?
Bitcoin as of 09/14/2013 has a global market capital of 1.618 billion dollars
What is a Bitcoin miner?
A Bitcoin miner is a contributing node to the global Bitcoin network (https://en.bitcoin.it/wiki/Mining). Mining computers around the world construct the Bitcoin network; verifying transactions and generating new Bitcoins.
Profitability of Bitcoin mining is based on the speed of Bitcoin mining units, minus investment & operating costs (the hardware itself, power, and cooling).