As the name might suggest, WikiHow is a wiki how-to guide, which is a populated by users of the site and powered by MediaWiki. It's essentially an extension of eHow, which itself has tutorials on a variety of subjects (and--actually--eHow has a lot more of them). As with any how-to, writers put together steps, tips, and warnings on any topic, and--in this case--other users can make additions or edits to those articles.
Like other wikis, wikiHow will let you search for articles, edit articles, and add your own articles. The site is very straight-forward and simple to navigate. You won't have to pay to use it since it is ad-supported, and what's even better is that those ads are quite nonintrusive. To add or edit an article, you will have to go through a simple registration process.
The service has more than 50,000 how-to articles that are constantly being updated and edited. They're usually found in the results from major search engines, which has led to a steady month-by-month growth since the service launched in early 2005. If you're looking for simple instructions, it's not a bad place to start.