Discussions

Good evening folks:  
I'm in the market for a new(ish) phone. I kind of like the Motorola i580 flip phone (sorry no link). The reason why is I'm really tough on my phones. I also need big buttons to dial with too. I don't need a smart phone..I don't think I do. I want to have my phone make phone calls (real one to one talking)and maybe 20-30 texts a month.  
 
My wife has a Blackberry curve 0$ (with a 3 year plan) with MTS. Having around 200-250 min a month would suffice me too. I want to buy my phone probably on-line(read cheap)maybe even used.  
 
So here are my actual questions. What does a phone that's "unlocked" really mean? What is a "Sim Card" and, what does it do for me, and my phone? Can I buy a phone to serve my purposes and use MTS as my server without a contract that wasn't bought at an MTS store? What are the pros and cons of going the route I wish to go?  
I'm looking to get a robust useful phone cheaper than buying one new at a store and, use that phone on my terms.  
 
Thanks