Differences between CDs and DVDs

If I buy a CD or DVD in Japan, can it be used in the US? If I buy a CD or DVD in the US, can it be used in Japan?

CDs will work fine since they are not region locked. As for DVDs however you will need a region free dvd player if you want to play DVDs bought in Japan on your DVD player in the US and vice versa (some exceptions not withstanding). Japanese DVDs are region code 2 while US DVDs are region code 1. Using VLC Media player you can play all region code DVDs on your PC dvd drive without problems though.

CDs are universal so they work the same everywhere.

For DVDs Japan is Region 2, the US is Region 1. So you need either a "region free" dvd player or a dvd player with the proper region coding to play the DVD.

If you want to play the DVD on a computer, there are many software tricks to allow a region 1 or 2 dvd to play on either.

I use a program called "DVD43" actually it is more like a plugin and it works fine on my computer.

Just FYI if you use Windows 7 it will lock down a region at some point.. it's possible to switch back/forth 2 or 3 times but no more.