A Wiki is a linked collection of publicly-editable web pages - any visitor can click the "Edit text of this page" link at the bottom of a page to amend or expand its text, if they've got something to say. (Edits are attributed to the IP address of your computer, by default - if you want to log in and give yourself a name instead, click "Preferences".)
You don't need to know any HTML or anything to edit a Wiki page - just write in plain text and the Wiki will sort it out for you. (There are also a few simple tricks you can do, like adding an asterisk to the start of subsequent lines to create a bulletted list - a full list of formatting tricks are [documented here].)
Wiki pages can be linked to one another - if you surround some text with double square brackets, when typing it in (eg. "[[Example]]", it's interpreted by the Wiki as being a link to the page with that title (eg. Example). (If it doesn't recognise the title, it adds a question mark link (eg. Example2?), which any later visitor can click to provide text for that title. This is also how you can add new pages. Titles of nonexistent pages that contain multiple words are shown inside single brackets to show which words are in the title (i.e. [No Page]? rather than No Page?).)
If you want to play around with editing a page without fear of breaking anything (and don't worry, it's impossible to do any damage - full change archives are kept, so we can get back anything that's overwritten), then feel free to edit the Sand Box.
The nature of Wikis means that from time to time random spammers will come along and add blatant links to their sites, but this quite easy to fix.
If you "View other revisions" of a page, you can bring up the revision before the spammer attacked and edit that revision. It'll have an "Editing old revision X. Saving this page will replace the latest revision with this text." explanation at the top - just click Submit without making any changes, and this old revision will be pasted back in as the new one, in place of the spam.