It's Basically like that:
You have one big sea with many Islands on it, and that seas is divided into the four parts by Grand line and Red line.
Grandline is basically another sea, but as you might've guessed it doesn't follow normal rules.
Redline is a mountain.
Grandline itself is divided into two parts: first part(don't know the name ^^) and new world.
Normally you get to grand line through loose mountain cause you can't get through Calm Belt (wonder if Wb can though, Marine can [Seastone])
When you are in Grandline itself there are seven ways you can choose to get to rough tale(last island in Grandline), but you see that pretty good
here
At the other intersection of Grand and Redline there's the holy land of Mariejoa, which is the main base of the marine, and the place where the WG was found.
What is in New World is still a mystery but oda has to reveal it soon.
Anything else to add???