Related: Tax consequences of short term capital losses
The net capital loss is a deduction, not a credit, and it is a separate item (Schedule D) from your standard deduction, meaning you can take the standard deduction and also deduct up to $3,000 in capital losses.
The nature of the loss (short-term or long-term) is used to calculate your net capital gain.
If you have long-term gains in excess of your long-term losses, you have a net capital gain to the extent your net long-term capital gain is more than your net short-term capital loss, if any.
- IRS Topic 409
- IRS Publication 550
- littleadv's answer to above-linked question