kobra:
Standard html coding for href and img src
^^^ This :D
I do exactly this to add extra links relative to the product, like links to the actual fitting instructions for an item so the buyer can preview them prior to purchase, etc so they have a fully informed purchase.
Then also add detailed parts photo showing dimensions to clarify the items specifics exactly. These are often related to part of the description, so adding them as main item photos would not make as much sense to the buyer using the site.
I also embed item relative videos to make the page more fun and interactive for the user, whilst still also being more descriptive.
Then I also add pull down menus for model fitment as well, else the page length would be huge on some items.
You can really do a lot with the description area if you know only a few basic HTML tags tbh.
What is a totally cool benefit is that the stylesheet information carries on with your descriptions. So if you add a link, it picks up the CSS and gets styled to suit the rest of the site by itself :D