Details about Cryptocurrency and how to report it can be found on this IRS tax publication NOTICE 2014-21
My understanding is that if you buy Cryptocurrency and sell it for a profit within the year it is a short term gain.
And if you hold it longer than one year and ultimately sell it for a profit later it is a long term gain.
The two types of gains are taxed differently and the capital gains taxable income amounts change subject to new tax laws.