The jersey that a fan wears can show a lot about them. Not just the team they like, but much more, down to elements of their personality.
Part of this theory comes from the idea that many fans are limited in the number and type of jerseys they can buy as they may be faced with the choice of buying a new jersey or buying food.
Please do not take anything personally, everything here is a generalization.
Alternatively if you do not know what type of fan I am referring to, Bryn Swartz's 10 most annoying fans article can serve as a good guide.
Star Offensive Player
The fan who goes out and buys the star offensive player jersey will likely know that this player is the best in the NFL at his position (regardless of whether or not he is). This fan will either know every stat about this star player, and will repeat it at every opportunity to prove how great they are, or will be a bandwagon fan who only has the jersey because he recognizes the name on the back.
This fan will likely tell you that their favorite team is better than yours because of, you guessed, the guy whose name they wear.
Star Defensive Player
This jersey is usually worn by a fan who is obsessed with the idea that defense wins championships. They will tell you that your offensive team would never beat their team because the number on their chest cannot be beat. These fans are just like the star offensive player fans description above, just replace the word offensive with defensive.
The Troy Polamalu or Ed Reed jersey would satisfy this condition.
Star Player on a Different Team
This is different from above, the player isn't on the team the fan says they have always liked. This is similar to the fan who roots for an individual player because they like them, but are not a fan of the team they play for.
This is a bandwagon fan, but in some ways much worse.
Until he was no longer a starter, Vince Young was the stereotype of this condition.
Greats of the Past
There are several types of people who wear this jersey. With that being the case I will sub-categorize this classification.
People Actually Alive at the Time
These people bought the jersey when the player was actually playing, and as such has been a fan for a long time. They will know everything about the team back in the day and will likely believe that even with more success, no current team could touch the greatness of the past.
This is only accurate, however, if they maintained fandom through the likely drought in greatness, if they jumped ship, they are bandwagon fans.
People With Teams in a Rut
San Francisco I'm looking at you.
Fans will buy their idea of the greatest player in team history when the guy currently replacing him isn't getting the job done. This is a better, and much more respectable, alternative to being a bandwagon. This fan wishes they were part of the glory days and are waiting it out until they come back around again.
People Looking to be Cool
This jersey can be just like the star player jersey. It is people who try to say that their number is the best number ever to play football. They get an old jersey to say that they have always been a fan of the sport and always knew who the best was. This is essentially a bandwagon who doesn't want to look like one.
They use this jersey to say they have been fans longer than you because their guy was on the team before yours.
A Terry Bradshaw, Joe Montana, or Troy Aikman jersey could fill this profile.
This jersey is bought for one of two reasons. Either the fan knows this player is good, and is a great fan of this player (or team) and knows success will come one day, or they don't want to be labeled as one of the above fans. Bandwagon fans typically shy away from these jerseys because they don't know who they are.
These fans either knew who the player was before they got to the team and know they will help the overall effort, or know the player is overshadowed by the greats on their team and "pays them respect" by buying their name and number.
These are typically true hardcore fans. This player must be on one's favorite team, however, or the fan belongs in the category above.
I cannot give a typical example of this jersey, as there is not one.
This fan is also an "either/or" case. This fan is either a very hardcore fan who cares more for their team than food/gas/anything else that costs money, or they want you to think that they are.
This fan will usually have jerseys from many of the above categories, as they have more than one jersey. Even though this is the case they belong in an entirely different category. This fan truly loves their team (or wants you to think so) and you should never doubt it (or you should).
There is no typical example of the multiple jersey fan.
Your Team's Jersey
This is the fan who knows what team you like, and buys a jersey to match this team. This is the fan who uses your team to become a better friend of yours. It usually backfires and causes you to hate them. If you are considering doing this, I would only recommend this if you want someone to dislike you.
This fan, though they think they are sneaky, is very obvious. You know they actually like another team, as they said so when you met them (or conversely didn't like the sport). Then one day they show up in your team's jersey. They talk about it all the time.
They seem to use it as conversation starter. Then the next day, you see them rooting for their old team (or not watching the game), but still talking to you about your favorite team.
Again, this category has no typical example, but this fan will usually buy a star player, or past great jersey, because if their attempt to become your friend doesn't work, at least they get a cool jersey out of it.