See here to know about PSA's https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=10035 Today blogger blogs are having only PSA's due to some bug: http://www.smartbloggerz.com/2009/01/adsense-not-showing-ads-today.html
Ads that are for suffering people,Money collected through such links are donated to charities by Google.
They are not bad in and of themselves because they tell you that what you are doing is not profitable. However there are times when Google runs them through sites just because they can! Depends on how they value your site.