What are hooks and define types of hooks in WordPress? code example

Example: wordpress most usesfull action and hooks

<?php
add_filter('body_class','browser_body_class');
function browser_body_class($classes) {
  global $is_lynx, $is_gecko, $is_IE, $is_opera, $is_NS4, $is_safari, $is_chrome, $is_iphone;

  if($is_lynx) $classes[] = 'lynx';
  elseif($is_gecko) $classes[] = 'gecko';
  elseif($is_opera) $classes[] = 'opera';
  elseif($is_NS4) $classes[] = 'ns4';
  elseif($is_safari) $classes[] = 'safari';
  elseif($is_chrome) $classes[] = 'chrome';
  elseif($is_IE) $classes[] = 'ie';
  else $classes[] = 'unknown';

  if($is_iphone) $classes[] = 'iphone';
  return $classes;
}
?>