In a nutshell, these are what Ubuntu's archive divisions mean:
1) main
: Free software, officially supported by Canonical
2) universe
: Free software, NOT supported by Canonical
3) restricted
: Non-free software officially supported by Canonical (includes device drivers mainly, amongst others)
4) multiverse
: Non-free software NOT supported by Canonical (flashplugin-nonfree comes over here)
Debian has these divisions:
1) main
: All free software that follows the DFSG (Debian Free Software Guidelines)
2) contrib
: Free software that follows DFSG but depends on software in non-free
.
3) non-free
: All kinds of non-free software that doesn't follow the DFSG.
Since Debian doesn't differentiate among packages on the basis of support (since all packages are supported by the Debian community), contrib
and non-free
packages correspond to Restricted
/Multiverse
in Ubuntu. By default, all contrib
and non-free
packages enter Multiverse
when they are synced. If Canonical intends to support them, they are moved to restricted
.