The main difference is the default window manager. Xubuntu and Lubuntu are usually used with older and less powerful computers. In detail:
Ubuntu comes with Unity a shell running over the GNOME Desktop environment. Unity is mean to be easy to use for most users.
Kubuntu uses KDE as the default desktop environment. KDE has got a lot of configurations and graphic effects and is good for power users who want to costumize a lot their environment.
Xubuntu uses XFCE a very clean desktop environment. XFCE is easy to use and uses less system resources than KDE and Unity (Gnome).
Lubuntu uses LXDE as the the window manager. LXDE is designed to work well with computers on the low end of the performance spectrum such as older resource-constrained machines.
All the versions of Ubuntu share the same packets and packet manager (apt-get), and they releases follow the main Ubuntu release (every 6 months). The release name is composed as year.month, so for example 12.10 is the release of October 2012.