baseLayout.jsp is the file upon which all other AppFuse JSPs are based. It contains the overall structure of each page, where the contents of the page are inserted via Tiles.
A peek into web/WEB-INF/tiles-config.xml will reveal that baseLayout is a name in a <definition> tag that points to the actual file baseLayout.jsp in a path attribute. Within the definition, certain variables are "put" into the "definition" with <put>, <putList>, and <add> tags.
Taking it in sections, the beginning on the file includes /common/taglibs.jsp, so each JSP that is rendered will have available to it all the tag libraries that are mentioned in taglibs.jsp.
Similarly, after that, /common/meta.jsp is included. This contains all the meta tags your pages might need, so you can maintain them in one place.
Finally, on line 55, we come to the <body> tag. Here, you can see that the bodyId string that may have been defined in tiles-config.xml is included as the id of the body, if present.
Next, the errors.browser.warning string is retrieved from ApplicationResources for the current Locale.
Following that, within the div named screen, the div named header is displayed. This section checks to see if the current URL contains the string login.jsp, because this header text is only output if NOT on the login page. This header section is where you find the logged in user's name, as well as the link that says "Logout."
Following this, the menu is output, with a deceptively simple <tiles:insert> tag.
Equally minimalistic, the next div is called "content" and prints out the heading as an H1 header, followed by the /common/messages.jsp, which will contains whatever messages have been placed in there by prior processing, and finally, the actual contents of the page is inserted with line 84's <tiles:insert attribute="content"/>.
At the bottom of the page, the footer defined in tiles-config.xml is inserted.