Seriously, what is identity?

A quick search in Google finds 204,000,000 results for the word “identity”. While browsing through the results, I stumbled upon a webpage that claimed to “sell” identities (“Peoplefinders - Where you can buy someone's identity, almost.”), three different dictionaries (and three different meanings), a movie released in 2003 called “Indentity” and a Youtube video that showed what apparently is a cat suffering from an identity crisis. By the time I finished visiting all the results I mentioned above I found that identity has been defined in many different ways, some authors define it in five words, while other authors need an entire 200+ pages book.
“Sometimes we feel that we straddle two cultures, at other times, that we fall between two stools.” writes Sir Salman Rushdie, the Indian novelist behind Grimus (1975) and Midnight Children (1981). This phrase could describe, in a way, how I feel about my identity. Sometimes I feel as if I spent more time in the United States than in Mexico. Let me elaborate on this: I spend most of my free time in front of a computer surfing through different American websites, studying from books written by American authors, playing online games in American servers... hey! I’m even typing this in an American computer! Even though I physically live in Mexico, it seems to me as if I’m virtually inside the United States. This, I believe, is because it is so easily to find a place online where people share your passions. Say, for example, your passion is listening to Green Day, all you have to do is load up your preferred search engine and type: “Green Day fans”, or “Green Day forums” and you will find mostly American communities of over 500 persons dedicated to extract as much juice as they can from the latest Green Day news, single, album or MTV appearance.
This has many persons, including me trapped between two walls: Am I Mardil (my Internet pseudonym), who likes to watch funny videos and read news on technology? Or am I Omar? The ITESM first semester student, who likes to go out with his friends, plays the bass in his band and loves going to concerts? To answer this question I asked my brother the following questions, two different days. The first day I asked him:
Me: Hernán, who is Peter Parker?

Hernán: He is Spiderman.
Then the next day I fired him this question:
Me: Hernán, who is Spiderman?

Hernán: Peter Parker.

Me: Is Spiderman more Peter Parker than Peter Parker Spiderman?

Hernán: I think Peter Parker is as much Spiderman as Spiderman is Peter Parker, because they are both the same person, under different names.

The answer came pretty clear to me after that: I can have a million of names, I could be “Jose Santiago” when answering online surveys, “Mardil” when posting comments on youtube, “Pedro Picapiedra” when being interviewed on the phone, “Omar” for my friends or “34234569” for teachers in my college, but in the end I’m not a name, but a person.
Some people need five words to define “identity”, some people need to add words to define it, such as “identity crisis” or “identity theft”, other people need to write entire books to really find what it really means; but identity isn’t defined in words, but in time. Some aspects of my identity will probably change with time, such as what I like to do and what I spend my time on, but other aspects (like ascendance) will remain forever untouched.