Ive noticed some players have some letter beside their name.

List and what i think they mean:
(i) - Inactive and weak, therefor i cannot attack. System wont let me.
(n) - Active and who i can attack and are similar in points to me so same rank area.
(s) - Umm...i dont dare, they look strong^^ For now:D
Nothing - When there is nothing i can attack...possibly same meaning as n, but just inactive?
* - Recently attacked? But sometimes i get this sign on players that i havnt attacked or done anything...
Red - Thats me:D but...why are some other players red? Thats not me:(

Or does n stand for noob or newbie?

What is the CORRECT meaning?

there is a legend if you hover over the word legend on the bottom right of the galaxy map

ah thank you^^ I didnt think it was a link.

Hmm but what does empty mean? Some players have nothing...? Also, some players have n, who i can attack and i cant attack?

red=no attack. 5 times stronger than you. you die.
green=sometimes attack. see if you can probe them. if you can then attack away.
nothing(white)=similar rank. you can attack, but probe first to see what youre going against.
red planet=your planets.
green planets=ally planets.

That is correct.