DVDs are in MPEG-2 format, so technically every DVD player can read that. Depending on the players BIOS it may or may not be able to read SVCD (MPEG-2), VCD (MPEG-1, but all MPEG-2 decoders can read that too), or plain MPEG-1 or MPEG-2 files that haven't been authored for any of the formats I already listed. Some players can even read MPEG-4 (Divx, Xvid, etc,...), but if you have a model that does that you should already know it.
Besides reading the user manual (which may not list all supported formats), a good resource for information on this is the database at DVDRHelp.com:
http://www.videohelp.com/dvdplayers
Just remember to take any information there with a grain of salt, as it's all submitted by users and therefore may or may not be correct. Plus, some DVD players, like those made by Apex, have different physical models that are sold with the same model number (or the same model sold with different numbers). In the case of my Apex, there are even multiple BIOS versions, which means that some of them will play formats that others won't, even though they have the same hardware.
Depends on the player, most players list the different formats a unit can play on the box it comes in. If your not sure then do an internet search before buying.
