A lot of new elements were introduced with HTML5 including footer, header, aside, article and section. The W3C HTML Working Group that presently creates HTML has accepted a proposal for the <main> tag to be added to HTML as well. If you’re really nerdy, you can read through the documentation about the “main” element.
So where does fit in, and what will its purpose be? The easiest way to describe it is that it will wrap around the main content. Main content being identified would not only be easier for markup purposes but would identify content similar to a “reader” mode like that offered in Apple’s Safari. Identifying “main” content could also exclude non-main content from a search.
I am personally in favor of the tag being added to HTML proper, and think it is a lot less ambiguous than article, section and aside, which all have a lot of conflicting information on how they should be used. I’m curious how the logic that got “aside” and “article” through the cut to HTML5 also concluded that “main” was superfluous?
HTML5 is about to reach the stage where no new elements can be added, but may make it into HTML5.1 which is on track to be finalized by 2016.