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Since Version 2.5 WordPress support so called Shortcodes. They have been introduced for creating macros to be use in a posts content. For examples of shortcodes and how to use them, see WordPress.com Shortcodes, though some shortcodes featured are exclusive to WordPress.com.
A trivial shortcode for a gallery looks like this:
You can also print a shortcode directly in a template like so:
<?php echo do_shortcode('[gallery]'); ?>
Shortcodes can be with additional attributes as the following example shows:
[gallery id="123" size="medium"]
Both examples will display an image gallery which would be hard to maintain when writing the HTML markup for it and keeping it in sync with uploaded images.
WordPress does offer some of the shortcodes by default (for example the gallery one) and plugins can add their own as well via the Shortcode API.