What software do you use and what file format do you use if you want
to save a movie file to a DVD-RW disk so you can play it back on a
regular DVD player on your television set?

Any good burner program contains dvd authoring nowadays , a decent
program for little money would be Nero Burning Rom

There are heaps of great ways to do this.

The easiest is to use Nero to create a 'DVD Video'.

I suggest you check out www.afterdawn.com for some great tutorials on
how to do this... for example:

http://www.afterdawn.com/guides/archive/convert_avi_to_dvd.cfm
This explains how to put a DivX or XviD AVI onto a DVD.

Some good tools can also be downloaded from www.doom9.org

As a first step though, it's important to know the file format that
you want to burn onto DVD. (ie. the Audio and Video codecs that are
being used).

With any Macintosh that you purchase, you should have iMovie
preinstalled and free of charge.  That will give you good-quality
video editing capability.  For $49, you can get the full iLife suite,
which includes iPhoto for digital photography, iTunes for downloading
and playing music, GarageBand for creating your own music, and iDVD
which will do the burning.  Not a bad investment, is it?  GO
MACINTOSH.

www.dvdrhelp.com is the best site for all info on what you need,
including how to burn dvds, svcds, and vcds

dvdSanta(http://www.dvdsanta.com) is the software that convert DivX,
AVI, WMV movies into DVD video format. It can load AVI, MPG, DivX, WMV
format video files and turn them into DVD videos which can play on a
stand alone DVD player. It can burn the DVD video into DVD-R, DVD+R,
DVD-RW, DVD+RW discs.