Differences between ECN and FDM

I have been looking into opening a forex account but cant decide which company to use. To add to me confusion,I read about something called ECN yesterday. I tried doing some research on it and i just kept seeing the same things but with no explanation. I read that ECN’s have prime brokers and they deal directly with the Interbank. Can someone please help clarigy this for me? Thank you very much.

Also a lot of the genuine ECNs are not available to US residents currently.