Ubuntu Linux is a popular distribution, and deservedly so, but it uses the GNOME desktop. While many like GNOME, there is no denying that it is a heavyweight among desktops, offering plenty of features but demanding of system resources. There are many that would trade features they don't use anyway for a lighter and faster desktop, especially if they use older or slower hardware.
Xubuntu is a standard Ubuntu distribution but with the GNOME desktop replaced with XFCE. XFCE is a full desktop environment, but one designed to place far less load on the system. It still has most of the features of the bigger desktop environments, and is an integrated environment, not just a super-lightweight window manager. For the majority of usage, you would notice the difference, except that XFCE is more responsive. This is especially noticeable on lower powered hardware, whether that is a netbook/nettop or an older computer rescued from the recyclers.
As both GNOME and XFCE use the GTK toolkit, all the Ubuntu preferences and administration tools work with XFCE too, so the experience is not that different. If you really need all the features of GNOME you should stick with Ubuntu, but try this and you may be surprised at how much better it runs without losing anything you actually need.
If you have a less powerful computer, or you just want a more responsive desktop, give Xubuntu a spin.