newbie question about packages

[nemir nemiria]

Hi.

Just trialling Ubuntu for the first time (usually I use *BSD, or Fedora if I 
need to run linux kernel...).

I am a little curious about the packages.  Not totally up with the history 
here, but it seems that Ubuntu is closely related to Debian,  and uses the 
same package management stuff.  The thing that is confusing me at the moment 
is whether I can use the debian packages under ubuntu or not...?  Some 
packages seem to indicate that I can, others don't.

Sorry if this has been covered off before,  but I did a (rudimentary) search 
for info and couldn't really find anything informative.

Nemir