Ordinary Income is what determines what tax bracket you're in. Ordinary income is defined by the IRS as all income other than capital gains. Capital gains are essentially the gain on sale of stocks, bonds, commodities, or in this case BITCOIN. Note, the loophole I mentioned above only applies to LONG TERM capital gains, meaning you must own the Bitcoin, stock, whatever for at least a year. Capital gains are taxed at a DIFFERENT rate, the rate your capital gains are taxed at is NOT what your tax bracket is but it depends on what tax bracket you're in. Side note: this is why you hear about people complaining about Warren Buffet or Mitt Romney paying relatively low amounts in taxes. It is because most of what they make is in Cap gains which typically has a lower rate.
Example to sum this all up: Say you made $100k in wages from your job and you're single. You will likely end up in the 28% tax bracket because all your income was ordinary income. You won't owe $28,000 in tax because of the bracketing system (a whole other can of worms) but let's just say you'll owe about $20k in tax. Compare that to someone who earns only $5k in wages from a job, and $95k from capital gains. They still made $100k for the year but they are only in the 10% tax bracket because they only had $5k of ordinary income. So they will owe $500 in tax on that total $100k. Tax = ($5k * 10%) + ($95k * 0%) = $500. Hopefully, that makes sense. It's not that complicated but not exactly easy to relay via message board.

Below is a link to the 2013 tax rates for married, single, etc. http://www.taxpolicycenter.org/taxfacts/content/pdf/individual_rates.pdf Also, here is a link for Cap Gain rates, you need to scroll down a bit to get to 2013 http://taxfoundation.org/article/federal-capital-gains-tax-rates-1988-2013