Please, do not confuse the word free meaning free of cost.
With respect to free (open source) software, free has the same meaning as free speech or freedom.
You have the freedom with free (open source) software to do what you want with it --- share it with friends, modify it yourself, give your modifications back to friends, etc.
Usually, this free (open source) software is also free of cost. Free in the sense you usually use the word. But this doesn't always have to be the case.
Regarding Ubuntu: it is not an offshoot, but a distribution of the Linux system, packaged as their developers see fit. It is in fact an offshoot of Debian, another (older) Linux distro.
Ubuntu is currently about the most popular distribution of Linux, and with good reason: it's been designed for ease of use, to open up Linux to a wide audience.
If you want to try it out, I'd definitely give it a go. It's easy to download a CD image, or just order one for free in the post, and try it out without having to install it on your hard drive.