So what's the difference between "social-democrat" and "liberal"?
I think there is a difference, although in practice both philosophies often support the same kinds of government policies. I think the philosophy of liberalism stresses individual liberty while the philosophy of social democracy stresses the need for the well being of the working class.
That's like saying what is the difference between an apple and a fruit?
There is none. "Apple" is a subset of "fruit".
I think you are thinking of libertarianism which is not the same thing.
These definitions are frequently covered in Political Science 101 where there is a clear separation made between "liberalism" and "social democracy." Unlike liberals, libertarians believe in small government. Both liberals and libertarians stress individual liberty. Social democrats believe in developing socialism through current western democratic institutions. That's why the term "democracy" takes precedence over the term "socialism." So the NDP are social democrats not democratic socialists.