Text ads are pretty much what they say they are: ads that have no graphics on them. The leader in text ads is Google; when you do a google search, text ads show up on the right-hand side of the page.
On the front page of
Kuro5hin , look at the upper left-hand side of the page; they've made it clear that the box there has advertising in it. That up there is your most complicated text ad...
These ads take almost no bandwidth and are unobtrusive and simple. You can easily ignore them if you don't want to look at ads. And for the most part they are bought by people with other, similar sites who just want a little attention, or people who want to support the site.