Let me preface by reiterating that I'm not trying to produce an authoritative definition, or devalue others' definitions (particularly the definitions that Christians themselves hold). I'm simply letting you know what definition I am normally working with when I call someone "Christian".
So here it is:
I take to be a Christian anyone who uses the teachings of Jesus of Nazareth as a guide to leading an ethical life.In practice, this definition will catch pretty much everyone who self-identifies as Christian. Of course, many "Christians" under this definition will not be considered True Christians by many other "Christians" (if you follow me).
I can live with that.
It also means that "Christians", under my definition, are a varied bunch indeed - Gnostics, Catholics, Fundamentalists, Mormons, Quakers, some Unitarians, and others. So varied, in fact, that I should be very wary of making sweeping generalizations about "all Christians". You'll tell me if I slip up, won't you?
Here are some other definitions on offer for what a Christian is:
OED (access to dictionary not free): "Believing, professing, or belonging to the religion of Christ." (This is pretty vague - depending on how restrictively you define "the religion of Christ", this definition is almost circular.)
Wiktionary: "An individual who seeks to live his or her life according to the principles and values taught by Jesus Christ." (Basically what I said.)
Dictionary.com and Cambridge Advanced Learners Dictionary: They each give a range of definitions, including something close to mine.
And of course, I encourage you to explore the discussion of this issue at the Ontario Consultants on Religious Tolerance.