Capital Gains Canada & US

Canada:
Capital gains are given favourable tax treatment in Canada.  
Only half (50%) of the capital gain is taxed all at your marginal tax rate (which varies by province).
Capital losses can be carried back three years and carried forward indefinitely.

USA:  
Capital Gains are only given favourable tax rates if the gain ​is on an asset held over ONE year.   So, if you sell a stock in less than ONE year, the tax would be the same as though the income were wages - there is no benefit.

If you hold your Assets longer than one year, you can benefit from a reduced tax rate on your gains. For 2015, the long-term capital gains tax rates are 0 %, 15%, and 20% for most taxpayers.

US Net Capital Losses are carried forward and a little bit of the loss can offset other income (eaten up) each year - $1,500 for Married Filing Separately and $3,000 for Married Filing Jointly.   If you still have excess capital losses, you can keep carrying them over to future tax years for as many years as you need to.

USA: 
If your Modified Adjusted Gross Income (MAGI) for US tax purposes meets the thresholds then you may be subject to a 3.8% Net Investment Income Tax (NIIT) which is the lesser of your net investment income for the year and your MAGI.    If you meet these thresholds, multiply your net investment income by 3.8% and this may be the amount of additional tax. 

Married Filing Jointly or Qualifying Widow(er) 
 $250,000
Single or Head of Household
 $200,000
Married Filing Separately
 $125,000