Elgg is a social networking framework. It provides the necessary functionality to allow you to run your own social networking site, whether publicly (like Facebook) or internally on a networked intranet (like Microsoft Sharepoint).
To run Elgg, you need to have your own web server and a certain amount of technical knowledge - or access to someone who does, like a system administrator. (There are more detailed requirements over here.)
There is also a demo site running a standard installation of Elgg.
Elgg comes with advanced user management and administration, social networking, cross-site tagging, powerful access control lists, internationalisation support, multiple view support (eg cell phones, iPhone), an advanced templating engine, a widget framework and more. A full list is available here Features.
If you need more functionality than what Elgg offers out-of-the-box there are a couple of options:
Elgg is an open source product. Not all open source products are free to download, but Elgg is. That means, in its simplest form, Elgg costs nothing at all. This does not mean it's public domain: Elgg is released under the GNU Public License v2, which means the source code is licensed to you. With any software, it's always a good idea to read the license.
Yes. Elgg is open source so you can change anything about it that you want. This includes removing the graphics that refer to Elgg. What you cannot do is remove the license file and copyright information.
Truth, justice and freedom!
No, in all seriousness, Elgg doesn't stand for anything. It's named after a town in Switzerland. However, if you prefer this answer, it also means "moose" in many languages.
Elgg's initial funding was by a company called Curverider, which was started by David Tosh and Ben Werdmuller. Today, Elgg is a fully open source product protected and sponsored by The Elgg Foundation and has a variety of contributors.
Elgg is written exactly like that: the 'E' in uppercase and the rest in lower. It is also acceptable to be in all lowercase. Elgg isn't an acronym, and there's no need to shout when writing, so please don't write "ELGG." E-LGG is also wrong.
No. "I'm using the Elgg" is a weird sentence. Similarly, "Why not just install an Elgg!" sounds funny.
If "Elgg" is used as an adjective to describe a noun that requires an article, you would use one. For example, in "I'm using the Elgg website," the word "website" needs an article, so "the" is used before Elgg.