How do I find and email to "Organic" email addresses ie subscribers?

How do I find and email to "Organic" email addresses, ie ones who have subscribed to our email list?

I have various email lists that I send marketing email out to. However I never get offered the chance to email my "Organic" email addresses, ie ones who have subscribed to our email list via the form on our website.

How do I a ) see who is  has subscribed to our email list? and b) how do access that list to email to it?