Rankings as you might guess rank people for doing an action. There are 5 types
1. Military
2. Economy
3. Battle Ranking or BR
4. Kills
5. Alliances
Realm wide or world wide[]
You can see how well you are doing in your realm, or the world.
You could be first in your realm in military, but how would you stack up to the world?
Level based in your realm or the world[]
You are also able to compare it by level. Usually you do this world wide as how many level 36's are in your realm 1, maybe 2 ?
Doing it by level worldwide to me seems perfect you can see how you preform against others but not feel completely dominated by those powerhouses.