What do I do when my Verizon contract ends?

This isn't specifically Apple, but I thought it would be good to post here.

I have a 4S on Verizon and my contract ends in December. When that's over with, I want to use my phone on another service so that I can lower my monthly bill (instead of upgrading). For anyone who has experience doing this with Verizon, what happens exactly at the end of my contract? (I'm new to this). Can I cancel anytime after my contract is up without a penalty? Also, is there any auto-renewing that I need to be aware of? Additionally, I'm wondering how the whole unlocking process fits into this, and any other the steps I would have to take to make this transition.

I'm new to phone contracts, and this information seems hard to come by for me. I'd appreciate any help. :)