If you follow the standard conventions,
edit.php will be used for the form layout that allows you to edit an item, and
default.php will be used for viewing an item as well as viewing a list of items.
Typically in Joomla, you will see two types of view folders: singular and plural. The plural version (let's call it "articles"), will just have a
default.php file that will render a list of articles. The singular version ("article" this time), may have both the
edit.php or could just have
edit.php. (Sometimes the default isn't implemented.) If both exist, the
default.php file would render the article information to be viewed but not edited. The
edit.php file would render the form allowing you to edit the article.