Its audience is all speakers of English in the world. General - Differs from formal English in that its audience may be all speakers of American English. Thus it can contain idiomatic expressions, colloquilaisms, and cultural references. An idiom is an expression not directly or easily translatable to another language. In neither case is the expression logical or obvious.
Colloquialisms are local ways of expressing things.
It would be appropriate to use the American colloquialism. The more regional the colloquialism, the more casual the tone of the writing becomes. Cultural references are things like historical events or personalities which would be familiar to an audience of most Americans. Informal - Addressed to people you know. The main characteristics of informal writing are abbreviations, sentence fragments, and other forms of ellipsis.