What is it used for?
NOTE: Use of the Wikipedia talk pages (the "discussion tab") is recommended as the first step in Wikipedia:Dispute resolution.
On Wikipedia, the purpose of a talk page is to help to improve the contents of the main page, from an encyclopedic point of view. Questions, challenges, excised text (due to truly egregious confusion or bias, for example), arguments relevant to changing the text, and commentary on the main page are all fair play.
Wikipedians generally oppose the use of talk pages just for the purpose of partisan talk about the main subject. Wikipedia is not a soapbox, it's an encyclopedia. In other words, talk about the article, not about the subject. It's only the habits we encourage that keep Wikipedia from turning into another H2G2 or Everything2. See also: Wikiquette
That said, Wikipedians are fallible creatures, so it's entirely natural that a bit of partisan wrangling takes place on talk pages - and occasionally this even leads to improvements in the article! So there's a fair degree of tolerance, and most Wikipedians succumb to a bit of wrangling from time to time.
The practice of "spamming" - posting similar messages to more than a few users' talk pages, often for the purpose of soliciting a certain action - is discouraged.