Well operating systems are either based off of 32-bit or 64-bit architecture, which defines how they handle information - the number of bits in each pass (essentially). Windows XP, Vista, and Windows 7 all have 32 and 64-bit versions. The bit-size is defined by the OS manufacturer - and then there is essentially two versions of the OS, one in 32-bit and one in 64-bit. The Windows 7 DVD will have both 32 and 64-bit versions on the same DVD, and you get to choose which you want to install.
I am not sure how it worked for Windows Vista, but with XP you had to buy a separate installation CD for the 64-bit version, and the default (masses) version was 32-bit.
I am not familiar with Debian, but here is some information regarding the 64-bit version. So it appears that there is both a 32-bit and 64-bit version of Debian.
Edit: Links always help: http://www.debian-administration.org/articles/534