Beginner D

" Grade Buff: Once you hit level cap you gain a different kind of level called grades. Theres 20 different grades in total.5 divisions :Recruit, Junior, Senior, Veteran, Master and they all split in 4 different sub-divisions: D, C, B, A,.As you level those grades you gain a global exp buff applied to all your characters going up to 100% with Master A rank. " - Nekavrad

^

Basically to make it more easy to understand

You start as recruit D and you level up till you are recruit C, all the way to Recruit A

then you 'promote' to Junior D and you do the same process as you did with recruit

and that repeats itself all the way till Master A which is the max you can be

(Names for ranks might be different on this server)
To level up your rank you need the usual exp from killing mobs and doing quests.

Is there any purpose to these ranks? like do we got more skill points, stat boosts, or anything?

Account wide exp bonus.