How it affects you is the IRS taxes capital gains. It taxes short-term capital gains (investments you've held for a year or less) differently thant long-term capital gains (investments you've held for more than a year).
Short-term capital gains are taxed at your regular income tax rate. If the Bush tax cut is extended, then you don't have to pay taxes on long-term capital gains if you are in the 10% and 15% tax brackets.Even if you make more, you only have to pay a 15% tax. Therefore, it's usually better from a tax point of view to hold onto your investment for at least a year.