List all the taxonomy terms in a particular vocabulary in Drupal 6.
You can use taxonomy_get_tree()

Parameters

$vid: Which vocabulary to generate the tree for.

$parent: The term ID under which to generate the tree. If 0, generate the tree for the entire vocabulary.

$depth: Internal use only. Now deprecated and isn't used. It is left here only because of compatibility issues.

$max_depth: The number of levels of the tree to return. Leave NULL to return all levels.

API here