An online archive site is similar to a normal archive, which stores books and other forms of text for years, so people can view this past information in the future. The difference is that an archive site does not store books; it stores websites that have been taken down or substantially changed. Another type of archive website is an archived forum, where the posts are still up but participation is locked and no one can edit or add to the forum. This ensures that the information on websites will not be lost if the website goes down because of low traffic or because the webmaster is done with the website.
Archived websites are either older versions of a website or a website that has locked participation and is dead. A website specifically designed to archive websites will typically show all the different incarnations of a website. Most websites go through several major design overhauls and usually include different articles for the incarnation. This allows the website to remain current and constantly update its intended traffic with new information. The archive site will allow users to see all these incarnations and find content that may no longer be on the website.
The other type is a locked and dead site. This is usually a forum but also may be a regular website. For the former, a forum where user participation is high and constantly adding content, the forum may be dead because no one is participating anymore or because the service of the forum has been suspended. Many people may still want to know about the topic, so the information is typically archived so users can read the information without it all going to waste. For a regular website, the content that was added to the website will be archived just like a book in an archive.
An archive site may try to categorize all websites or just store specific online information. For the former, it will attempt to get all website incarnations and will store every website it can, which results in high server costs for the webmaster but also allows the archive site to serve the highest amount of people. The latter is only interested in a topic, such as health articles or content about business, and may store only the article itself or the entire website that contained the article.
There are two reasons for archiving a website. One is because useful information is constantly posted to the Internet, and archiving a website ensures that the information is never erased from the Internet. The other is that there will typically still be traffic to the website and, if the website is able to make money through advertisements, this allows the webmaster to rely on the archived site financially without having to worry about upkeep.