The IRS classifies all cryptocurrencies as property. Buying Bitcoin is not a taxable event. But using Bitcoin to buy something else is considered a sale of Bitcoin and selling property for more than you purchased it for is a taxable event. If you "sell" some Bitcoin at a profit that you purchased within the last year, you will have to report short term capital gains on your tax return and pay ordinary income tax rates.
Now that the Supreme Court has made it easier for states to require online retailers to collect sales taxes, how will states and Congress respond? Some states may be able to set a collection regime by the end of this year.