All components should set the following values

$smarty->assign("title", "Page Title String");
$smarty->assign("body", "output of module");

Optionally,  you can set a right hand side panel
$smarty->assign("secondary", "side boxes");

You can turn on a meta refresh with
$smarty->assign("refresh", value);

Set additional stylesheets to be applied to screen output (strongly discouraged)
$smarty->assign("extra_styles", array('/path/to/additional/stylesheet.css'))

The master script supplies the following php variables:-

$path  		The path of the current selected page
$pathlist  	Broken out version of path in array

$component	The pagemap record used for the current page:
	[id]		Record id
	[path]		path of the root node for this component
	[depth]		path depth
	[component]	component name

$smarty		The smarty object
$DB		The ADODB database object
$session	The Session object

Session has the following properties
$session->loggedin	TRUE if logged in
$session->username	the username
$session->fullname	the users real name
$session->data		array for storing persistant data

any values stored in data[] will be persistant between pages
for as long as the users session cookie stays active.

For error handling, your component should use the trigger_error() function:

trigger_error("Informational message", E_USER_INFO); // To inform the user that something happened, e.g. the session expired
trigger_error("Warning message", E_USER_WARNING); // Warn the user of an expected error, e.g. incorrect password
trigger_error("Error message", E_USER_ERROR); // Reserved for unexpected conditions, e.g. unable to connect to database

You may also use the message_flash("status message") function for helpful messages, e.g. form submitted