Informal writing??

I've met the tasks to write an informal letter to a friend of mine or an information sheet.
But it's difficult to know whether it is formal that  what I've written.
What makes informal writing??
Now I'm trying to write in an informal way while I'm writing this entry.
If you feel something formal apart from being a light writing, let me know what it is.

I don't know what is exactly the difference between an informal writing and a normal chat on the web.
Would it be an informal writing if I use less grammar structure? (No by the way)
Or is there no rule to determine whether it is informal?

I might be used to writing in an formal way, so I can't tell which one is which.
Or I might be right.

Thank you for reading my entry.
Please correct some strange expressions or spelling errors.
I would appreciate (this is formal by the way, non formal: I would like it) it if you reply with your opinion about what I've concerned.