Whereas Reviews contain reviews of a particular game, Sessions contains Session Reports about particular games. Normally a Session Report will be about a single playing of a particular game, or possibly about a series of plays, but they should contain details about how that particular playing of the game progressed.
A session report is a form of submittable content. Session reports describe, in different amounts of detail depending on the style of the writer, what happened in a play or plays or a certain game. They are different from a review, as although a session report may contain some elements that are like a review, such as discussion of strategy and some player's first impressions, a session report is more focused on explaining what went on in a game, as opposed to explaining what will go on in a game. Session reports go through the GeekMod system when submitted.
To submit a session report, simply go to the forums of the game it is for, select Post, and in the "Select a Forum" pulldown menu, select Session Report instead of the General forum.
Your session report will be easier and more enjoyable for people to read if you write it well, paying attention to grammar and spelling. Reading Common English Mistakes may be useful.
Session reports are available on the the game pages and in the Sessions Module which is shown by default on the Front Page.