Where adware comes from
Just like spyware, adware is most often built into into free software, but can also be installed on your browser or operating system through a security hole.
How to recognize adware
Your device is probably infected with adware if you start seeing ads popping up in applications where you hadn't seen them before. Pop-ups can also show up on your computer's desktop even when you're not browsing the Internet. Also, your browser's homepage may have been changed. You can use an adware scanner (which is included in all adware removal tools) to check if your device is infected.