The most evident difference is the desktop environment. There are also different choices of software that are bundled in the iso file. It is possible to install software from the same repositories and this includes the desktop environments. So you can install Lubuntu-desktop with the whole package or only LXDE, the ultra light-weight desktop environment, or only Openbox, the window manager, which gives you an opportunity to play more demanding video or use more memory for the application programs, than with standard Ubuntu with Unity or Gnome shell.
You select desktop environment at the log in screen (click on the circular button near the password window).
sudo apt-get install lubuntu-desktop
sudo apt-get install lxde
There are methods to remove these things and restore Ubuntu. See this link
sudo apt-get install openbox
Before doing this, you should backup your system, because it is easy to break it but hard to restore it.
I don't know how much ManamiVixen knows about the future plans of Ubuntu, but usually the LTS versions are very stable. Things might stop working between versions, but very seldom within them. Ubuntu 12.04.3 will be very similar to the present 12.04.2, it will include some more bug-fixes and security updates. You can download and compare 12.10 which was released last October and the next one, 13.04, to be released this month.