What's the difference between "e" and "este"?
e is the abbreviated form - like the difference between She's and She is, he's and he is, or it's and it is. But its only used for este.
Actually "e" is just informal. That's what i found in the wiktionnary
Well, yes, it is informal. Just as "he's" is an informal version of "he is". But it's also, by it's very nature, abbreviated.