How to make certain words turn to links

I always wondered how I could do this. Say for example I have 10,000 posts on my blog, all about cooking, and one of those posts was about how to make vegetable stock.

How could I set something up so on any old posts or future posts, if ever the word vegetable stock was wrote that would actually be a link to the vegetable stcok page.

This I would vey much like. Also something that could be setup like any time the name of a cooking store was wrote, this could be an affiliate link for me which takes person away if clicked.

Is there any sort of plugin that does all this.

Much appreciated, thanks.

Shaun