Gnome, KDE and Unity are desktop environments. Basically, it's the GUI, but there are other differences too. Anyway, if you want to go deeper into the subject I included a wikipedia link.
Gnome - was the default DE (desktop env.) for Ubuntu until version 11.04
Unity - is the new default DE in Ubuntu 11.04
KDE - is the default DE in Kubuntu
In Lubuntu, the DE is LXDE.
You can run all programs in any DE, but some look better or might load faster depending on what DE they are designed for.
Your machine will have a hard time running DE's like Gnome, Unity or KDE.
So see if you like Lubuntu, if not, maybe Xubuntu (which uses Xfce as DE)