Your Internet Footprint is your "purposeful identity" on the Internet (Leaver, 2011).
What does this mean? Erving Goffman, in his work The Presentation of the Self in Everyday Life talks about what it is that makes up the notion of the self. It is the sum of all your experiences, and so your Internet identity is the sum of all your web interactions. Also the way people interact over the internet is a reflection of themselves. As people have different views of who they are, they often choose an avatar that reflects their own 'theory of self' so to speak.
Internet etiquette, or Netiquette, and our use of it, is a reflection of our personalities, and one needs to be mindful that everything we write on the Net shows a little of who we are and what kind of person we are. Many social networks have rules about netiquette you need to adheare to if you wish to be part of that network.
Leaver, T. 2011. iLecture Internet Footprints. Curtain University.
Goffman E. 1959 Presentation of Self in Everyday Life. Double Day Anchor Books Doubleday & Company,, Inc. Carden City, New York.