What is the difference between unlocked iPhone 6 and T-Mobile no contract iPhone 6?

So I tried to go on apple website and look for an unlocked version iPhone 6 Plus, but they only have T-Mobile version is without any contract, but it comes with a T-Mobile SIM card, does it means that I have to use this card and T-Mobile service? Can I use other carrier's card instead if I buy this version of iPhone? I travel a lot, like going back to China, I use China Mobile, can I use their SIM card on this T-Mobile iPhone?

I plan to build a simple robot using Raspberry Pi and Breadboard?

I want to build a simple robot using Raspberry Pi B+ and breadboards. I plan to build the robot's body using a 3D printer. But I have no idea what motors and wheels and stuff to use. So basically:

1. Body-3D printer
2. Processor-Raspberry Pi B+
3. Circuits-Breadboard
4. What do I need in a circuit?
5. What motors and cables do I need?
6. I also want to control the robot using my IPhone 4, could you tell me how to do this?
7. I have a very basic idea of electronics so could you point me to why I need what you are proposing. I have a computer engineering class and I'm learning bread boarding and stuff.
8. I want to use python for this project could you perhaps point me to a few tutorials regarding robotics and controlling physical stuff for beginners. I already am learning python programming myself, but I don't know how to apply that to hardware. I'm a total beginner and just started learning

Will my iPhone 4 work in 2015?

My cousin told me my carrier will no longer carry iPhone 4 in 2015. Is this true? I've looked it up and I can't seem to find anything about it

Iphone headphone JACK having trouble playing music?

Everytime i listen to music on my iphone 4s with headphones in i have to pull the headphone thing out alittle bit for it to play, i've tried to clean out the inside of the headphone jack with tape and stuff but nothing seems to work. Can anyone help?

I've tried many different headphones, its the headphone jack switch, when i put the headphones in it continues playing out the speakers.