IMPRESSIONS: Essentially the number of page views an ad slot receives. If one was to say that a website gets 1000 impressions a day, it means it gets 1000 page views.
UNIQUE VISITOR: If you have visited a page / website 5 times in a day you would not be looked at as 5 users, but 1 only. Your IP would only be logged in once. This would be reset every day.
CLICKS: A visitor clicking on a particular section on the site – be it an ad, an article, or anything clickable.
CTR: Click – through – rate. The number of people who click on an ad, on the site, per 1000 impressions. Hence if a site gets 20 clicks for a 1000 impressions then the CTR is 2% i.e. Total Clicks\1000 X 100.
CPM: Cost to the advertiser for every thousand impressions for his ads. So a 200 CPM would mean that the advertiser will pay the publisher 200 for every 1000 impressions his ad is exposed to.CPC: The advertiser pays the publisher for every click. If an advertisers is paying 5 CPC, then for every click that the ad receives the publisher get 5 of the relevant currency. So 100 clicks would be 500 in revenue. The number of impressions is irrelevant.
CPA: Could mean 2 things – Cost per Action or Cost per Acquisition. The former means that the advertiser expects the user to complete upto a certain level of interaction before the publisher will get paid….say not only going to a travel site but doing a search as well. Cost per acquisition refers to the cost involved to an advertiser to be able to acquire a user as a customer under some description. For a travel advertiser it could mean not only a search but a purchase of a ticket… or registering in a new website.
CPL: Again, the cost involved to generate a lead. Hence when a departmental store is willing to pay 200 CPL, it means that they will pay 200 for every user who becomes a possible sales lead for them. It does not matter if the user eventually purchases anything or completes the final goal.
EMAILER / MAILER: All those emails which you keep getting in your inbox. Well someone is sending them and someone is getting paid for that. If you have registered on a mobile forum. The owners can sell the registered user database comprising of people such as you to an advertiser.
VIRAL: A little difficult to explain in a single line. Will put up another chapter for it. You can see the detailed explanation here.