The dangers of adware spams

Hi and good day Techyv friends and pals!
I know adware is a software that can be downloaded to your computer to show advertisements that can take many forms like banners and pop-up windows. But here are my set of question, how can I get Adware on my computer?
Why does Adware exist?
What does Adware do?
What are dangers of it?
Can I get adware spam?
Any help would be nice and would be appreciated.
Thank you very much.
Kurtt Stewart
