At the top of the sidebar on the right, there are 3 tabs: Recent Comments, Recent Posts, and Archives:
When you click on Archives, you will see all the months for which there are posts. Click on a month and you will see the posts. By default Archives is highlighted. If you click on one of the other items and look at one of the elements, Archives will go back to being highlighted.
Recent comments simply lists the most recent comments. Click on one to see the details. When you do so, the Archives tab will be highlighted again by default.
Recent posts lists the most recent posts. Click on one to see the detail. When you do so the Archives tab will be highlighted again by default.
Similar information is available in the New tab in the upper menu.