You can set the Title and the Tag-line by setting the following in config.php:

$WikiTitle = "My site";
$WikiTag   = "Site description";

The wiki file Site.SiteHeader can be overridden with a Group level page. If you want to change the layout of the header edit either {$Group}.SiteHeader or {$SiteGroup}.SiteHeader. The default value for the header is:

! %block sitetitle%[[{$ScriptUrl} | {$WikiTitle}]]
(:div class="sitetag":){$WikiTag}