Column guidelines allow you display a vertical band within the code editor.  This is useful to help identify the maximum size a single line of text should be within your code (for example: to avoid wrapping when printing it out).

The power tool extensions enable you to add a column guide pretty easily.  Just cursor over in your code editor to where you want the guideline to be (note: you can see the exact line column count at the bottom of the editor):

Then right-click and choose the “Guidelines->Add Guideline” menu command:

This will then cause VS to show a column guide-line in all code editor windows at that column width – making it easier to see when you’ve written code or a comment that wraps beyond that:

If you want to remove a guideline, you need to move your editor cursor to be at the same column width as the guideline.  You can then right-click and use the “Guidelines->Remove Guideline” command to remove it.