Drupal nodeapi

Some notes about Drupal 6 node API.

• "alter": the $node->content array has been rendered, so the node body or teaser is filtered and now contains HTML. This op should only be used when text substitution, filtering, or other raw text operations are necessary.
• "delete": The node is being deleted.
• "delete revision": The revision of the node is deleted. You can delete data associated with that revision.
• "insert": The node is being created (inserted in the database).
• "load": The node is about to be loaded from the database. This hook can be used to load additional data at this time.
• "prepare": The node is about to be shown on the add/edit form.
• "prepare translation": The node is being cloned for translation. Load additional data or copy values from $node->translation_source.
• "print": Prepare a node view for printing. Used for printer-friendly view in book_module
• "rss item": An RSS feed is generated. The module can return properties to be added to the RSS item generated for this node. See comment_nodeapi() and upload_nodeapi() for examples. The $node passed can also be modified to add or remove contents to the feed item.
• "search result": The node is displayed as a search result. If you want to display extra information with the result, return it.
• "presave": The node passed validation and is about to be saved. Modules may use this to make changes to the node before it is saved to the database.
• "update": The node is being updated.
• "update index": The node is being indexed. If you want additional information to be indexed which is not already visible through nodeapi "view", then you should return it here.
• "validate": The user has just finished editing the node and is trying to preview or submit it. This hook can be used to check the node data. Errors should be set with form_set_error().
• "view": The node content is being assembled before rendering. The module may add elements $node->content prior to rendering. This hook will be called after hook_view(). The format of $node->content is the same as used by Forms API.

Source: http://api.drupal.org/api/function/hook_nodeapi/6

When interacting with other modules, the order that Drupal loads the modules is important. Here you find a SQL query showing the order of your modules.