What format do you want as an end result? If you have ripped a dvd-video straight to files ( in a Video_TS folder as Whappo points out), then you can use Vob2Mpg to get an mpeg2 file from them. The free version of Vob2Mpg works fine for this. This program just takes the video out of the vob container format, and leaves you the mpeg2 video file, with no loss of quality.
If you want a different format, or a smaller sized file, you can use a conversion program. Handbrake is a good one when going to H264 mp4 or mkv files. AutoGK is the old standby for xvid avi files. You'll lose a small amount of video quality when converting to these formats, but done well, you probably won't notice it in actual playback.