How to automatically turn certain words into links

I’ve been googling around trying to find a way to turn certain words (on a site i’m working on) into links. The purpose is **not **to create advertisement.

I’ve seen this kind of thing on lots of sites, which is why I was a bit surprised that I can’t find anything about it. The only thing I’ve found so far is a wordpress plugin called “link a dink” which does exactly what I want, except I’m not using WP (see

Does anyone have any idea?