Can someone tell me the difference between new construction windows and replacement windows? What do you measure for replacement windows?

New windows comes with a nailing flange to set the window from the outside.

Replacement windows usually set inside the old window jamb.

Here's some vids to look at the options

YouTube - Window Replacement Part 1

If you want to tear the old aluminum ones out, here's a video showing the tool you use to remove the old aluminum frame:


YouTube - Aluminum window frame remover - "X-Tractor"

New construction windows have standard sizes. Replacement windows are typically ordered to custom sizes. New construction are cheaper for this reason. Measure the window on the outside of the house, order windows 1/4 inch smaller. If you are planning to replace with vinyl windows, and if you can find new construction 1/4 inch smaller than your existing windows, you can score the nailing fin a few times on each side with a utility knife and break the nailing fin off.

I've also seen people just nail the old flange (fin) to the exterior of the siding and frame around it!