Plugin directory

Hi there. I’m new to Expression Engine, not to PHP/MySQL.

I want to develop a plugin that grabs data from a database and formats it. I would like it to be used in a template like this:

{exp:pluginName:specificMethod}
<li><a href="http://{url}">{text}</a></li>
{/exp:pluginName:specificMethod}

where “specificMethod” is some set routine querying a database, and where the row is looped for the number of records found.

I have a few questions.

1. Is it a plugin that I want? I don’t need to configure it or anything like that, but I read somewhere that a plugin is for template-type actions, and querying the database is hardly front-end. Would a module or extension be more appropriate?

2. Which plugin folder do I use? I see “system/plugins”, “system/expressionengine/plugins”, and “system/expressionengine/third_party”. I’ve read that it should be “system/plugins”, but the “Magpie RSS Parser” and “XML Parser” plugins that ship with Expression Engine are under system/expressionengine/plugins… The Hello_world plugin from http://expressionengine.com/docs/development/plugins.html that I tested with doesn’t seem to get registered no matter where I put it.

3. Somewhat related to #2. Is there some cache-clearing or setting-changes required to get the plugin to display in the plugins manager in the control panel? It’s not showing up on a browser refresh, and the editing templates gives a “plugin not found” warning.

Here is the source for Hello_world (direct from http://expressionengine.com/docs/development/plugins.html):

<?php

class Hello_world
{

var $return_data = "";

  function Hello_world()
  {
    $this->return_data = "Hello World";
  }

}

?>

The file name is “pi.hello_world.php”, and I tried placing it in “system/plugins”, “system/experssionengine/plugins”, and “system/expressionengine/thrid_party” with no effect.

Many thanks,

Anthony