Followers and Subscribers… the difference

Hi!
I’m a relatively new blogger and picking up things on the way. On two occasions, I randomly get a mail saying :
New user registration on your site Thoughts:

Username: xxxx

Email: xxxx
The second time, I had 7 mails!!

On checking the dashboard > Users, I see i have all the subscribers.

My question… how did they get to subscribe without my knowing or permission?? plus, to my knowledge, the only option I have is to subscribe via e-mail. ( http://www.anupal.org) When I check the Jetpack site stats, it mentions the number of ‘followers’

So is this spam? how do I stop it??