Off all the crazy accronyms out there, I think CPM is the most important.  Very simple, this is what you, as the advertiser, pays for 1,000 impressions.  Long long long ago before fire was invented (well not that long) CPM rates were well out of touch with reality.  As an advertiser, if you ponied up for a campaign you probably said to yourself, "...self....I think I got ripped off."  CPM rates of $20 or even $30 were not unhead of.

Rates these days can maybe see the mid teens, but more than likely you should be seeing something in the range of $8-$12 for good publishers, a little lower for smaller sites, and then $3 - $6 for vertical networks ( a vertical network is a collection of sites all focused on the same thing...Like a bunch of cool hiking sites would be a "hiking" vertical network).

Below this, there are a few levels even lower, but as they always say, you pay for what you get.  If you want to spend less than $1 CPM (we call this area "remnant"), it may be a great way for a large branding effort across the web.  While you may not be targeting the exact audience you want, at least your message is getting out into the wild and seen by millions.

So why wouldn't you do this all the time?  Think about it this way.  If you are a florist that only sells wedding arrangements, by taking an add out that reaches a wide audience, you reach a lot of people, but not all of them are getting married like next month. 

Its the same with remnant advertising.  You are getting the message out, in massive amounts, and reaching a balanced mix of the american public.  But what if you know that 8/10 times, a person who is in the market for a camp stove is usually in their mid 30's, makes about $40K per year and often lives out west?  Sure you are going to reach this person, but you are also going to spend a lot of marketing dollars reaching retired chess players in Florida. 

This is where targeted networks come in.  By focusing your ad dollars towards the right demographic mix, you are getting the best of both worlds.  You are not paying a premium CPM, and you are not scraping the barrel with a remnant CPM.  You would expect that as a result of this, your message is clicked on more often and the performance of your campaign should be better.