I'm just wondering if it's possible to put files onto a CD-r and play them in any DVD player.

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No. What your looking for is something called video-CD (VCD), or super video-CD (SVCD) (both require transcoding and hardware support)
 
The problem you would face from putting dvd vobs onto a CD would be the bitrate.

Well my friend did it a long time ago with butterfly effect. Why would the bitrate matter? How would it effect the playback? How can I put dvd files onto a CD-r and properly play it back on DVD players, such as a PS2?

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Just the way dvd hardware works.

Why put it on a CD anyway? DVDs are cheap

If your really adamant about doing this, look into SVCD.

YES, you can.
you just have to rip the dvd and then burn it on a cd. the only thing that yuo have to have in mind is that your dvd player must read divX.  
don´t bunr it as a video cd.
that is the way to go

Ok for the dumb dumb post above:
Divx (MPEG4 ASP), VCD (mpeg 1), SVCD(2600kbs MPEG2 with 2ch shound), and DVD have absolutely nothing to do with eachother (in terms of compatibility).

Simply transferring the vobs (MPEG2), ifo, bups... the dvd structure to a CD will not work "in most DVD players". Basically what the OP is asking for is miniDVD, although its even less accepted on players than SVCD.

its is clearly that you havn´t rip any dvds ever. riping is the best way to fit a dvd into a cd. or haven´t you see any torrent?

now how´s the the dumb?

you can find a lot of riping software in download.com
just make sure that the file don´t exced 700mb or if the movie is to long (the godfather....) you can split it in two cd´s

"Ripping" Is the process of copying (HD)DVD/BR Files from a disk and transferring them to your hard drive.

Transcoding is the process of taking the files that you just ripped and encoding them in a more efficient format to save space. Popular codecs like Divx, Xvid and x264 are just a few examples. Likewise, you can also encode the audio to other formats like vorbis, mp3 or aac.

Another way to save space is to use tools like vob blanker, or ifoedit to delete unwanted "features" like extra audio, menus etc... leaving just the main movie. This way is perferred as it causes no quality loss when compared to transcoding.

By the way, it is highly suggested that you not discuss illegal torrents/downloading movies around here.

Oh and yes, I have a massive collection of movies and anime that I rip and convert to mkv with x264 and vorbis.

EDIT: Please dont double (or triple for that matter) post.