What are the difference between add_filter, apply_filters, add_action and do_action?
I can see that do_action puts a tag in the code which can then be used by add_action. However, I can’t see how add_filter and apply_filters correlate with this. One would think that there would be a do_filter which would add a tag to the code, much like do_action; but neither of these seem to do this. And neither of them have a simple form like do_action( $tag ), but instead require callback functions as parameters.
Filters in wordpress are used to modify text. So, for instance:
Stripping SQL statements from a block of text before putting it in the database to avoid injection attacks. That would be a filter.
Actions are functions that are called when wordpress does something. For instance, (from docs), emailing someone when a post is written.
http://codex.wordpress.org/Function_Reference/add_action
http://codex.wordpress.org/Function_Reference/add_filter