Forum Moderators: open
"contextual advertising" - you do not look for something. You are just reading an article. We try to show you commercial relating theme of the article (AdSense is the best example)
"search-triggered" - you do look for something. You have typed your query in a SE so we show commercial relating your query. (Overture – is the best example)
by the way, imho it's obvious from the meaning of words :)