In app advertisement is taking over the advertisement world. In app ads are ads that pop up in apps you may be using that try to sell you their product or even service. You might be playing a game on your phone and an ad will pop up for a new car when the game is over. That is a very broad example of how an in app ad works.
There are many advantages to advertising this way. There is the fact that no matter who is using the app, they will see your ad, your ad is also very easily accessible when portrayed in an app. The disadvantages of having ads in an app is it can tend to annoy people. I know when I use an app I tend to get frustrated with pop up ads. They can also sometimes be eliminated with fully purchasing an app.
Mostly the younger crowd is the one that will be targeted the most with these ads. They are on their phone more than anyone and will be more likely to see these ads.