Shortcodes are a little WordPress magic that allow you to add really cool functionality without even needing to know any programming. There are essentially two kinds: [this-kind] and [this-kind] Stuff in between [/thiskind].
Place a shortcode inside your post or page editor and WordPress will replace it with the fancy code you don’t need to know.
The difference? [this-kind] replaces just the shortcode. [this-kind] stuff in between [/this-kind] replaces the shortcodes AS WELL as using what’s in-between them.
Technical details about all this WordPress Shortcode jazz can be found in the WordPress Shortcode API Codex. Warning: a lot of technical stuff there.