Send is a simple Microsoft app for sending SMS-like short emails. No subject lines, no signatures, it's more like an instant messaging program which happens to communicate via email.
Right now the app is only available on the iPhone, in the US and Canada, for people with Office 365 and school email accounts, but if that's you then it's easy enough to use. You can send messages to any of your Outlook contacts (they don't need Send or Office 365) just by selecting them and entering the text. That's it.
The recipient will see the message in Send, if they're using it, but it'll also arrive as an email and they can reply as normal.
Similarly, you can carry on the conversation in Send, if you like, but your messages are also synced with Outlook. That's the real plus here as it keeps your full emails and quick Send messages in one place, and means you can reply in whatever way is most convenient.
What's New in Version 1.2.29
Improved first run experience for new users.
Send is fairly basic right now, just a quicker way for North American Office 365 users to send emails, but that might change when (as promised) Microsoft makes it more broadly available. One to watch.