Core comes with a number of default template files. In order to override these templates, all you need to do is copy them into your theme folder and clear the theme registry.
You can also override these templates in a more targeted way.
This section lists the default templates in Drupal core and explains how to create more targeted template files.
In Drupal 6, some template files could be overridden in a targeted way. For example, the theme could contain a "node-article.tpl.php" file which would be used for nodes of the article type only.
In Drupal 7, these need to be renamed to use a double-hyphen. For example, "node--article.tpl.php". A single hyphen is still used to separate words: for example, "user-picture.tpl.php" or "node--long-content-type-name.tpl.php", so the double hyphen always indicates a more targeted override of what comes before the "--".
See Converting 6.x themes to 7.x for more info.