Summarize:

When you sell a stock for a profit, you are subject to what is known as a capital gains tax, or a tax on your profit. Profits can be taxed at two levels. If you hold your investment for more than one year, the profits will be taxed as capital gains at their tax rate, which is 15%. If you held your investment less than one year, they will be taxed as normal income. This means they would be taxed at the same level as your regular income as per your tax return. Tax loss harvesting refers to selling a stock at a loss, which can then be used to offset capital gains taxes on a gain. This is a powerful tool to use your losses to reduce your overall taxes. This means that if you sold a stock for a large capital gain this year, and also decide to sell a different stock (which may have lost money and be overvalued), you can use to the loss to offset the gain, and reduce your overall taxes. For example, assume you held a stock for over one year, and that stock increased in value from $10,000 to $17,000. If you sell that stock, you would normally be taxed $1,050, or 15% of the gain ($7,000). If you also had a stock that has lost a total of $5,000, you can choose to sell that stock to offset the $7,000 gain. In this case, your net gain would only be $2000, and you would therefore only pay $300 in taxes (15% of $2000). Short-term capital losses must first be applied against short-term capital gains (under a year), any extra can be applied against long-term gains (over a year). Long-term losses must first be applied to long-term gains, and then to short-term gains. If total losses for the year exceed gains, you can apply up to $3,000 towards reducing personal income taxes, and any extra can be carried forward to additional years to reduce future gains. A wash sale refers to selling a stock at a loss, and then re-purchasing the same stock, or a substantially similar stock, within a short time period. In this case, the IRS will disallow using the loss to offset gains. For example, you bought a XYZ stock for $100 in January 2014. On November 15th of the same year, you sell it at $90 and plan to claim the $10 per share loss on your income tax return. However, you hear a rumor that the company is selling very well going into the Christmas holidays after Black Friday so you buy the stock back at $88 on December 3rd. The government decides that you bought again because you expect the price to go up. They know that there was no real loss incurred. It was just a temporary exit from the investment which you plan to hold going forward. There is nothing that prevents you from buying and selling as often as you want, you just can't deduct short-term losses if you break the wash sale rule. If you have a stock that has lost money, you can consider selling it for tax harvesting purposes. There are, however, several considerations. Make sure you have good reason to sell any stock with losses, and make sure not to sell for tax-loss harvesting purposes alone. For example, if you have a stock that has lost money, and has become overvalued, this may be grounds to sell. Similarly, if you have a stock that has lost money, and you need to re-balance your portfolio, selling your losing stock can be a wise idea if the other stocks are still favorable.
Learn about capital gains taxes. Understand tax loss harvesting. Learn the wash-sale rules before deciding to sell. Sell a stock for tax harvesting purposes.