whatsapp runs in background and you don't need to sign in. There's no way to exit; you have to kill the process.

A message sent to you while you have no internet connection will wait to be sent to you again when you are online, pretty much like other instant messenger programs.

What's quite different from other im programs is that whatsapp identifies users by their phone numbers, not a username. After your friend in your phone book install and run whatsapp and you refresh your contact list in whatsapp, his/her name will automatically be present; no need to add friend by username or email address.

It is some what like the concept of enum (e164) where one identity (phone number) is used for every mode of communication, i.e. mobile phone, home phone, sip etc.