Here are all the levels with descriptions:

Very Low level: Weak/Average/Strong [VLL-W / A / S]
This is the lowest level of Reiatsu. Mostly people that have just begun spiritual training.
Examples: Humans who have yet to find their abilities, normal hollow, shinigami students.


Low level: Weak/Average/Strong [LL-W / A / S]
At this level the Reiatsu of your character has already developed itself to a Reasonable level, Your character is able to use Kidou and control their Reiatsu, your character will be able to use their special abilities better if they have any.
Examples: Humans who just discovered their power, larger hollow, shinigami that just gained shikai, class 3 vizards.


Middle Level: Weak/Average/Strong [ML-W / A / S]
This will be the most commonly seen Level, it's the average.
Examples: Humans that are skilled with their power, new arrancar, shinigami who have mastered shikai, class 2 vizards.


High level: Weak/Average/Strong [HL-W / A / S]
This is commonly known as the Captain level, this is a strong level that only strong characters will be graded with.
Examples: Humans who mastered their powers, ranked arrancar, shinigami captains, ranked class 1 vizards.


Very High level: Weak/Average/Strong [VHL-W / A / S]
A rank that is held for those who are truly gifted
Examples: Organization leaders

VERY IMPORTANT: The examples are only examples and are not set ranks. Depending on a characters app quality and posting quality, even a shinigami with a mastered shikai can be a very low level. Same goes for those with less power but a better app. A shinigami without shikai but an amazing app could be mid-level. This all depends on your app.

These are guidelines for the sub-levels:
Weak:
You're attacks are placed on a weaker end of the spectrum. You hit harder than those a tier below, but you have trouble fighting those above you.
Average:
These characters can stand up to those on both ends of their tier. While having more difficulty with Strong characters and less with weak characters.
Strong:
This is the strongest class you can reach in one level. You can fight any level below you, and only have some sort of trouble with those ranked at Average.

NOTE: Just like with ranks, if a weak vs a strong it is not just a set battle. The weak still has a chance to win and battles such as these will be monitored. Also averages of one ranks are capable of beating weaks of the next up. Anything can happen so don't get discouraged from any fights.

How you rank up:
Ranking up is determined by posting quality, contributions to the site, activity, etc,....
Staff will watch and see who demonstrates both the skill and activity to level up.