A talk page is a special SourceWatch page containing discussion about the contents of its associated "subject" page. To view the talk page of an article, click on the "Discuss this page" link in the sidebar or at the bottom of the screen. When you are in the talk page, clicking on "View article" will take you back to the main article.
On SourceWatch, the primary purpose of a talk page is to help to improve the contents of the main page. Questions, challenges, parked text, arguments relevant to changing the text, and commentary on the main page is all fair play.
Your user page has a talk page as well, and that one has some special features. For one thing, there is a link to it in the header next to your name (if you use a "skin" other than the default it may be somewhere else). Also, if edits are made to it by others, an asterisk (*) will appear next to the link to notify you. These pages can be used for occasional personal communication among users; but note that these pages are public. If you want to communicate privately, use e-mail.
A few community standards do apply to talk pages: for example, comments grow by adding text to the bottom of the existing discussion, so readers can follow the discussion chronologically. Unlike the articles themselves, comments on talk pages are often signed by their author. The software even makes that easy by letting you type three or four tilde characters (~, often in the upper-left corner of your keyboard), which will be turned into your user name if you are logged in (four tildes will produce your name and the date). There is also a "nickname" feature in your user preferences for customizing this signature.
Acknowledgment: the content of many of the help pages in SourceWatch have been adapted from Wikipedia.