There is a detailed explanation here and here. If you are really interested you can read the Debian Policy Manual also.
To answer your questions:
X = this is the debian version of the package
ubuntuY = this is the Yth ubuntu version of the debian package
To quote Verbatim from the first link, package-XubuntuY****” means:
package = this is the name of the program/library.
X = this is the debian version of the package
if X=0 this means that there is no debian package (or that the ubuntu
team has forked a debian package to a newer version than the one found
in the debian repositories) _ ex: bzip2-1.0.3-0ubuntu2_ (as shown in
this example the debian package might be updated in the meantime and
the ubuntu package will probably merged with it on the next version)
ubuntuY**** = this is the Yth ubuntu version of the debian package. if
this is missing this mean that it is a clean, unchanged debian package
ex: gzip-1.3.5-12 (in this sample, this is the original debian package
included in ubuntu) if this is present it means that Ubuntu has taken
the debian package and released it with some additional patches or bug
fixes. _ ex: sudo-1.6.8p12-1ubuntu6_ (in this sample this is the 6th
version of the ubuntu package based on the debian version 1.6.8p12-1
of sudo).
