Yeah, Union and Weld are the same thing, just different terms for different programs. The purpose of welding is so when you have things overlapping it still cuts as you want.
For example if you draw two rectangles and have them cross each other to create a cross what you see on screen will look like an X. But if you do a preview you'd see the two rectangles crossing each other so that when it cuts you'd have a square in the middle. So when you weld it and cut it, you'll get the X that you intended.