Quality Based Minimum Bids:

In the past Google had a 5 cent minimum bid price and only allowed ads to display
if their Google clickthrough rate was above 0.5%, but in August of 2005 they
shifted to a quality based minimum pricing.

This means that if your ad is deemed exceptionally relevant you can get clicks for as
low as 1 cent, depending on how competitors are bidding. Also if your ads are not
exceptionally relevant Google will still allow you to run advertisements, but you will
have to pay a higher minimum bid to make up for the lack of ad relevancy.

Ads which are disabled from search syndication due to low relevancy and bid price
still appear in the Google contextual ad program. I describe syndication later on in
the chapter.

Broad Match, Phrase Match, and Exact Match:

Google AdWords and Yahoo! both have different levels of word matching. Using
[search term] will only return searches for search term, which is called exact match.
Yahoo!’s exact match is a bit fuzzy, matching plurals and some common
misspellings. Google’s exact match is more precise, only matching the exact search. Using "search term" will appear on any ad that has search term in the query (in
the same order), which is called phrase match.

Using search term (no quotations or brackets) will appear on any search that has
search and term in it. With broad matching, synonyms to search term will also
return results. If you decide to start off with broad matching you should view the
synonyms to ensure none of them are wasteful. This is especially true with
acronyms or other terms with multiple well known meanings.

With Google there are all three levels of ad control, Yahoo! groups phrase match

and broad match in a category called advanced match.