What is the vertical line at the end of a measure called?

bar line

http://www.dolmetsch.com/musictheory3.htm

The vertical line at the end of a measure in music notation is called a "bar line." It helps to visually separate measures and indicates the boundary between one measure and the next.

To identify the bar line in a piece of sheet music, you can look for vertical lines that are thicker and extend from the top to the bottom of the staff. You will usually find bar lines between each measure, including at the very beginning and the end of a piece.