This thread got me thinking about common usage of these two words — salary and wage — and the answers provided were definitely in line with my experience with these two words, but then I got to thinking, after reading some of them, Haven't I heard "annual wage" used before? And then Do we ever say "hourly salary?"
So I paid a visit to Google's Ngram viewer and searched for combinations of "salary" and "wage" with "annual" and "hourly." Due to some comments made about American English versus British English, I also filtered for that. You'll see what I discovered in the image below:
For the record, I was pleased to see that "hourly salary" scrapes the floor of this chart because it sounds really off to me. As for the rest, you can see that there are some differences in usage. For starters, it looks as if Americans might talk about wage and salary a bit more than their British counterparts (or at least write about it a bit more). But other than that, the only real difference I noticed is that Americans might be more likely to use the phrase "annual wage" a bit more than Brits. In terms of just the two words alone, however — salary and wage — the word "wage" seems to be taking a sudden turn downward. Note the decline of its use in books for both American English and British:
As I mentioned earlier, I agreed with most, if not all, of what I read in this thread, but I have not yet found an authoritative source that strictly limits "wage" to "blue collar" work. As just one example, I took a look at the Wikipedia page for wage as well as wage labour and nowhere on it do I see any mention of "blue collar," but the page for "wage labour" ("labor" in American English) does include this:
... the wage work arrangements of CEOs, professional employees, and professional contract workers are sometimes conflated with class assignments, so that "wage labour" is considered to apply only to unskilled, semi-skilled or manual labour.
I can't know what native tongue a reader of this might speak, so I don't know how well "blue collar" translates for you, especially since the term originated in Iowa, but Wikipedia has an entire page devoted to it that I think most would find interesting. You can link to it here and it's been translated into several different languages, which may be helpful to some.
From personal experience, I know that a guy who works in a bar typically earns a wage, but if he accepts an offer to become a manager, he then makes a salary. What's the difference? With a wage, you get paid for every hour you work; with a salary, you work as much as is needed with no change in pay no matter how much you work. In the restaurant world, I've occasionally heard managers complain that they made more money working for hourly pay. Restaurants aren't the only place where one might go from an hourly wage to an annual salary. It's also common practice in offices and it's usually a real "feather in the cap" to go from an hourly wage to annual pay. It typically means that the powers that be in a company like you and like your work and want you to become a more permanent member of the company. However, even though that annual salary comes with higher pay and more prestige, it also comes with more responsibility, greater expectations, and more of one's time, as many have come to find out too late.
The TL;DR to all of this is that while "salary" and "wage" are pretty interchangeable in common usage, this isn't always the case. It's one of those things that you'll just have to learn as you go and the best way to do this is simply by getting as much exposure to the English language (through both the written and spoken word) as possible.