How to create automatic internal links

I’ve noticed, for some reason on my website that the words ‘video production’ are automatically turned into a link which goes to our About Us page. The only thing is, I don’t remember how I did this. I’d like to create more internal links such as this however am having difficulty in understanding how to make that happen.

For example check out: http://www.headstudios.com.au/2011/09/23/phone-quotes-quote-online/

If you search for the term ‘video production’ you’ll see it’s a link, I did not do this, it was created automatically. I would like to make more of these, for example ‘seo blogging’ or whatever.

Any idea how I would do it?