Recieved clicks but no earnings

Many of you may see clicks with no corresponding earnings in your AdSense reports.the reasons could be one or all of the following:
1.The clicks have been deemed as invalid by Google.If this trend of invalid clicks continue your account could be banned forever.
2.The ads clicked might be the CPM ads.Now let me explain the 2 different types of ads.One is the CPC ads(Cost per Click),where you are credited each time someone clicks on your ads.Another one is the CPM ads (.Cost per 1000 impressions).Impressions refers to the number of times the pages containing the CPM ads are loaded.The ads by themselves may or may not be viewed.In case of CPM ads you will be credited with a small amount even if no visitors click on them,as it is based on number of impressions and not on clicks.
3.The other reason is PSA ads.Public service ads are ads from non-profit organization that are served to pages by Google when targeted ads are unavailable, or or when it is unable to gather content from the web page.In such cases you are not credited with any earnings,for clicks made on PSA ads.