Behaviours: Private, intimate communication, normally with only one or two others, often using private references, slang or code
Expectations: Absolute privacy and control over the communication between users, and no unauthorised communication from third parties (eg spam)
Examples: SMS, IM
Please do not teach people, kids especially, that SMS and IM give them absolute privacy and control. They don't, and I'd especially avoid the word "secret," unless I was teaching some applied cryptography at the same time. I don't know the law in Scotland, but in the US at least, the current precedent is, by my non-lawyerly understanding, that it is "reasonably forseeable" that anything a kid says on IM is not going to be kept secret and can be held against him or her in disciplinary and legal proceedings.
But even on a more basic level, sending messages across a network that can be logged both by the IM or SMS service and by the recipient is just not a good way to keep a secret, compared to, say, picking up a land line or meeting face to face.