the_content

the_content might be the most essential WordPress hook. It is a filter on post content, run on display. It would be a good idea to take a look at wp-includes/default-filters.php to see what filters WordPress is applying, and with what priority. For example, wpautop() is run at default (10) priority. If you hook in at priority 11, you'll get <p&gt</p> tags. If you hook in at priority 9, you'll have \n\n line breaks.

Context:

File: wp-includes/template-functions-post.php
function the_content($more_link_text = '(more...)', $stripteaser = 0, $more_file = '') {
	$content = get_the_content($more_link_text, $stripteaser, $more_file);
	$content = apply_filters('the_content', $content);
	$content = str_replace(']]>', ']]&gt;', $content);
	echo $content;
}

Context:

File: wp-includes/functions-formatting.php
function wp_trim_excerpt($text) { // Fakes an excerpt if needed
	global $post;
	if ( '' == $text ) {
		$text = $post->post_content;
		$text = apply_filters('the_content', $text);
		$text = str_replace(']]>', ']]&gt;', $text);
		$text = strip_tags($text);
		$excerpt_length = 55;
		$words = explode(' ', $text, $excerpt_length + 1);
		if (count($words) > $excerpt_length) {
			array_pop($words);
			array_push($words, '[...]');
			$text = implode(' ', $words);
		}
	}
	return $text;
}

Context:

File: wp-includes/functions-post.php
function do_trackbacks($post_id) {
	global $wpdb;

	$post = $wpdb->get_row("SELECT * FROM $wpdb->posts WHERE ID = $post_id");
	$to_ping = get_to_ping($post_id);
	$pinged  = get_pung($post_id);
	if ( empty($to_ping) ) {
		$wpdb->query("UPDATE $wpdb->posts SET to_ping = '' WHERE ID = '$post_id'");
		return;
	}

	if (empty($post->post_excerpt))
		$excerpt = apply_filters('the_content', $post->post_content);
	else
		$excerpt = apply_filters('the_excerpt', $post->post_excerpt);

This hook is a filter which means that information is passed through it, and then used by WordPress. Your function needs to accept that information, and return it. Using add_filter('the_content', 'your_function'); helps you to remember this distinction. When you are passing an ID, it is assumed that you will return the ID as it was given to you. With filters that pass strings or arrays, you may manipulate the information before passing it along.

This hook is filed under Filter, Posts.