You can edit some online encyclopedias such as http://www.wikipedia.org and http://icklepedia.com
Others, such as Encyclopedia.com don't and are edited by official authors.
The free encyclopedia that anyone can edit.
Wikipedia
Wikipedia is a free encylopedia that anyone can edit. It will ban you from editing for a certain amount of time if you cause vandalism.
Numerous people contributed to the Wikipedia biography of Stan Lee. Wikipedia is a free encyclopedia that anyone can edit.
encyclopedia
Some popular encyclopedia sets include Encyclopedia Britannica, World Book Encyclopedia, and Encyclopædia Universalis.
A wiki is a collaborative website which allows every logged-in member to edit the pages and contribute to the content of the site. Wikis are mostly used for documentation or encyclopedia types of sites as far as I know.
Encyclopedia Britannica.
No, an encyclopedia is a secondary source.
Wikipedia is an online encyclopedia while a plain encyclopedia is on paper.
It isn't possible to say how many pages are in any one encyclopedia without knowing which encyclopedia, and which edition of that encyclopedia, is of interest.
No. However, the coincidence in names provides a fun tidbit. A wiki is defined as "A collaborative website which can be directly edited by anyone with access to it" - both WikiAnswers and Wikipedia are plays on this word: WikiAnswers is a site where anyone can ask, answer, and edit questions, whereas Wikipedia is a wiki-encyclopedia which anyone can read or edit.