Let's take a minute to explain what pay per click is, pay per click is when you pay for visitors to go to your website, most websites get their traffic from search engines, and spend a lot of money optimising their website to make sure it ranks highly in the top 10 on Google for example, however with Pay per click advertising you don't need to do this, the traffic is instant the only downside is it can be expensive and is a difficult art to master, if you do get it right it can be very effective and profitable for your website..
Pay per click advertising is done on a bid format your create your ads and then place bids, for example advertisers bid for slots of advertising space on either webpages, categories of specified niches specified by the advertiser, for example when you register a campaign it will ask for your bid for the advertising space you want, the higher your bid for this space the more preference it will get, and your advert will be displayed more often than lower bids. You can of course change your bids at any time, to get higher or lower preference you can also control where you want to target your ads, by country language and even what time you want your adverts to display this allows you to target your PPC traffic effectively.
CPM ads are impression ads, the difference being is that you will get 1000 impressions or page views for a specific price, the problem with impression ads is your paying for your ad to be shown on the page, but the downside to this is it doesn't guarantee you a click on your advert, as you are only paying for your advert to be shown and not clicked. However it is usually cheaper to pay per impression rather than pay per click, you can also target your impressions in the same way you can with PPC.
Flat rate adverts - For example a website owner will say you can have an ad spot for a specific price for a specific period of time, It would usually be a month or a week, so you pay for this spot for a flat rate. And get a guaranteed advert spot for the price and period of time you have paid for.