These are examples of each tier and what it could be compared too. Obtaining a tier doesn't mean you are comparable to the examples in every way or even a few ways. It simply means you have the potential to fight on their level.
Level 0: Leaders Rank
This rank is held exclusively for Organization leaders. If an organization leader drops their rank, they will be demoted. Unlike the other Tier ranks, the 0 tier has no sub divisions, cannot be bought, and will most likely change hands quite often depending on activity. Certain Event characters who lead an event have the possibility of being treated as 0 tier as well during the event. But their followers will not and upon the finishing of the event ( Completion or Inactivity killing it ) they will lose 0 status even if they survive the event. These people can handle several 2 tier characters at once and can even fight 2 tier 1 characters at once and be at the advantage in both cases even if their abilities are at a disadvantage. Most likely winning the fight in either instance as well.. These people are to be feared because they didn't attain a leader position for nothing.
1st level: Captain/High Rank
Tier Level 1: This rank is for masters of their respective race. This is the highest pinnacle of power anyone ( Excluding Organization leaders and certain event leaders ) can attain. Those at this level have access to the full might of their power and are extremely effective with them even in disadvantaged situations. Vasto Lorde and Captains of Gotei 13 who have been such for centuries generally fall into this rank.
Tier Level 2: This is for characters who are more powerful than 1-3, but not quite 1-1 yet. Ichigo would fall into this rank as he isn't as "experienced" as century old captains..but has the potential power of one.
Tier Level 3: On this level, your character is on par with seasoned captains. Captains with great skill and great control and knowledge of their bankai would fall into this level. Some examples of this level are Byakuya Kuchiki and Grimmjow Jeagerjaques.
Tier Level 4: This is for characters who are more powerful than 1-5, but not quite 1-3 yet. Adjuchas transformed arrancar fall into this category.
Tier Level 5: Here is where you are on the same level as new Captains. Newly appointed Horsemen, Human Teachers, and such. Examples of this would be Mayuri Kurotsuchi and Aaronierro.
2nd level: Lieutenant/Moderate Rank
Tier Level 1: This is a level for the rare vice-captain who has obtained their bankai. Renji Abarai is a good example of this Tier. Adjuchas class Menos would fall here. Elderly sins of the Apocalypse would also be classed in this group.
Tier Level 2: This is the tier level for a vice-captain who has mastered their shikai, but has yet to learn their Bankai. Rangiku Matsumoto is a good example of this.
Tier Level 3: This is for characters who are more powerful than 2-4, but not quite 2-2 yet. Experienced sins of the Apocalypse would also be classed in this group. Human students with a good mastery of their powers would also be here.
Tier Level 4: This is a tier level for vice-captains Who have mastery over their shikai, but aren't even close to learning their bankai. The average vice-captain would fall into this category. Gillian class Menos would also fall here.
Tier Level 5: This level is for newly appointed vice-captains with a great mastery over their Shikai, but haven't fully mastered it's use yet. Marechiyo Omaeda would fall into this category. Beginner sins of the Apocalypse would also be classed in this group.
3rd level: Officer/Low Rank
Tier Level 1: This level would be for a highly seated officer who has good knowledge of their shikai. Yumichika would be an example of this class.
Tier Level 2: This is for characters who are more powerful than 3-3, but not quite 3-1 yet.
Tier Level 3: This level is for the middle class seated officers. Most of these officers know their shikai, but have just learned it recently.
Tier Level 4: This is for characters who are more powerful than 3-5, but not quite 3-3 yet.
Tier Level 5: This level is for the lower seated officers, who can hear their zanpakuto's voice but cannot hear it's name quite yet.
4th level: Member Rank
This is the Unseated Shinigami,Humans that can see spirits but have little to no powers, Garden variety Hollow, etc. It's the most basic Tier level. As with the 0 Tier, this category will not have sub divisions and cannot be bought. Getting this is next to impossible as most people will do better than this no matter what.