Nat Type Fixes

I have wireless connection for my xbox 360 and I noticed that my NAT type is closed. I often have problems with my connections such as lag. On Halo 3 it gives me a message saying my NAT type is closed so matchmaking may be slow.

On Call of Duty 4, I often get that problem where it gives you a message after a round that you have lost your connection even though it is seems to be still working and then you lose all your experience you just gained. Also, when playing COD4 with my friend over live, I get connection problems and bad lag.

So here are my questions:

1. What is NAT Type?

2. Are my Call of duty 4 problems related to this?

3. How do i fix these problems (open my NAT type)?

Thanks so much!