Hi / Hello :)
Can any one tell me if there is any difference between Hi and Hello?
I would say that in British English in particular, 'Hi' is more informal than 'Hello'. I would say it to people I know better like my friends and also to younger people. If I am meeting someone for the first time, speaking to much older people than myself or professional people. I would use 'hello'. We use both frequently though.
In addition to the comment below, I would say that the tone of voice also matters in the case of hello. So something "Hello-o" (with the rising of the voice on the last o) is like asking a question in the sense of getting someone's attention.