With Snippets you can make reusable snippets (pieces of text/content) and include them into your content via a tag, like:
If you edit the article (or module or wherever you put the tag), you will still see the tag. But on frontend view you will see the content of the snippet instead.
You can use this to insert much used content for instance. Then, if you change the content of the snippet, it changes everywhere you have used the tag.
You can pass along 'variables' with the tag which can be placed inside the snippet. This way you can use one snippet and it will generate different output, depending on what you pass along in the tag.
How to use it: http://www.nonumber.nl/snippets
NoNumber extensions require php 5.3.13+
See for more requirements: http://www.nonumber.nl/snippets#requirements