I'll try to answer this as how I believe you are intending:
The applications that you download are likely ad-supported,much like when you browse on your computer.
You can rid most,if not all of these by checking the apps settings for ad permissions,uninstalling the apps that don't have this option,or,purchase the PRO version of the affected apps,which usually removes the ads from the apps.
If you are willing to root your phone,you can install an ad blocking app that should take care of the issue.
