Are my wages, on my 2018 W-2, for wages earned in 2018, or wages paid in 2018? In other words, if all my Dec 2018 wages were paid in Jan 2019, are they in my 2018 W-2?

Your 2018 W-2 includes wages paid in 2018 (assuming that your employer filled out the W-2 correctly). If you were paid in January 2019 for work that you did in 2018, that payment will be included in your 2019 W-2. Of course, your 2018 W-2 might also include wages that were paid in 2018 for work that you did in 2017.