I am so confused about SAG?

ok i am so confused about this whole actor guild thing? what is SAG? how does it help actors? Do I need it if I want to find an agency or if i want to audition for films? How do I join if i wanted to? When I go on the SAG Website to find agents this pops up: Franchised Agents ATA/NATR Non-Franchised Agents...very confused about tht. What does tht mean , can i just audition for agencies or do i need to be part of SAG