How exactly "Subscriptions" works?

I still do not understand very well how different 'subscriptions' work within the site. Does anyone know?

For example, what does the 'subscription' in a profile?

What does the 'subscription' in a group?

What does the 'subscription' on anything within the site?

What is the difference between you become a friend of another profile and have only 'subscription' activated with this profile?

What is the difference between a profile click 'join' in a group and just click 'subscription' of a group?

I still do not understand. Does anyone know?

Thank you.

A vague guess/explanation-

Subscriptions enable a person to "Follow" activity without becoming a friend or joining a group.

I guess it would almost be like- "Hey- here's my email address....Send me a message anytime something changes with this person or group....But don't tell them I am watching" :P LOL

I suppose that's how it works?