RFC destinations types ??

I am a beginner in XI. I am trying to do a scenario IDOC to file.

I don't get an idea on the following RFC connection types.

1. What is R/3 connection type and when we use it?

2. What is Logical destination type and when do we use it?

3. What is TCP/IP connection type and where we use it?

Any help would be greatly appreciated.

Thanks,

Sailaja