The most common way, is to transcode them into a more efficient format.
Before I refer you to anything, just let me tell you that this is a road to a long time consuming process. But it can be fun and is really easy once you get the hang of it!
For a general reference, www.doom9.org... read up on everything you can. I can sum it up pretty quick though:
Rip the main movie (many different methods and tools)
Transcode video to x264, XVID, Divx (x264 is recommended) You can also run filters on the video, like a nice denoiser, and deinterlacer, color correction etc...
Transcode audio to AAC/Vorbis/Mp3
Rip subtitles
Mux audio/video/subtitles into a container like MKV or MP4.
search for the program "megui" and read a few guides if you can find them. It provides a nice front end gui for many of these tasks
You can essentially take a ~6gig movie, improve it with some good filters (final output can look better than the dvd source!) and turn it into a ~1.5gb mkv file