Within the default design the title you see in the title bar of your browser is controlled by the title module. You can change the site wide default title the title module displays go here in the admin tool:
PAGE MODULES > MISC > TITLE MODULE
While browsing categories that module will display that categories title. While browsing a specific listing that listings title will appear in the title. There are configurations to set the title for the "front page" and "all other pages".
You can easily remove this module from use and set your own site wide titles by replacing the title module tag in the head.tpl template with a title of your own.