What is Disa?

It's actually #disa, or d.i.s.a depending on the context, but both represent the Usenet group set up by Demon Internet, called Demon IP Support Amiga which helped Amiga computer users get on to The Internet.
This Website is the web equivalent of the Usenet group and the IRCNet channel, and was born around November 1998.
The actual Usenet group was born around January, 1996.

Right, so what is Usenet?

Usenet is alive and well since its birth in 1980 (before the Web). In simple terms, it was (and is) a system which you could send data (text and images, like an email) to, and this data would then be sent to lots of other people who opted to receive it.
It is divided up into news "groups" which is a good thing otherwise you would receive a lot of data and a few disagreeable images.
Check out Wikipedia for a more thorough explanation.

And IRC?

Easy one this. It's like a MSN/Facebook/Skype group chat but simpler. It has also been around for ages but now tends only to contain geeks and strange people.
IRCNet is a subsection of the global IRC network, and came to life when two groups of people had a big fallout.
It was also the playground of hackers who would spend their time trying to destroy (at best) your connection or (at worst) your computer.

And what is an Amiga?

It's an old computer whose arch enemy was the Atari, back in the days when Windows based PC's were even worse at everything than they are now.

Demon?

They are one of the oldschool UK Internet companies. They became THUS plc sometime ago, and are still going.
A lot of the most well respected geeks have worked for Demon at some stage.

Do "Demon Still Support Amiga"?

Well, the group is still there... But we imagine anyone still using an Amiga would be such a guru they would need little help or support.

And you are.....

contra (Simon Britton). I've been around for ages too. I prefer FreeBSD and Ubuntu these days, but still appear on #disa regularly along with a handful of others.

And the Internet is..?

Go away. Now.