This.
There are 4 categories (in Canada) that you can not use to not hire someone.
Well, obviously you can, but if they ask why they didn't get a job you can't say why if it is in one of the 4 without major trouble.
The groups are:
1. Women
2. People with disabilities
3. Aboriginal people
4. Visible minorities
Sure, I can not hire a woman because I don't want a woman working in a job, but I can't say to her "Sorry, men only".
There are actually other things, but I don't remember them all (like religion, sexual preference, age)