Allow people to register on your site and you can set the default user role as subscriber or contributor. If someone is a contributor they can write a post, edit that post but it will not go live until you or an Editor allows it (I think Editors can, but don't quote me on that!).

I'm not sure what you mean about the excerpt custom field, or do you just mean the post excerpt. It's handy at times but I've used it more on CMSs than blogs.

About the custom fields I mean any visitor being able to send a post and it stays in moderation until an admin approves it.

I've been using Mini Forms plugin for that but would sure prefer to be able to do it myself.

I reckon that I have to create a default user to allow to create content but don't have a clue on how I would go from there.

Simply put, a custom field is a bit like having a second (and third if you wish) content box for that post. One of my examples of use was a meta description. Forget your SEO plugins for a moment because I don't think you need them for blog posts.

When you're creating a post you can set the post title and the post content. So if you created a custom field for the meta description, then in your template you could have the meta description field in the header.php file retrieving this meta description ie.

<code><meta name="description" value="<?php echo get_post_meta($post->ID, 'meta_desc', TRUE); ?>" /></code>

Then when you write a post you also go to the custom field area, in the key select list you'll see meta_desc (once you've set it once it will always be in the select list) and you type your description in the text area box. It's only assigned to that specific post then.

If that still doesn't make sense then I recommend you actually follow my post above and try it out for yourself. Then you should understand how it works.

-Mike