Ubuntu for phones uses Android as its base, but beyond that, is completely new. It has its own interface, and developers can write applications in Qt5 and QML, fully native OpenGL, C and C++, and of course HTML5. Like Android, it will run on both ARM and x86, and (some) existing devices will be supported as well. Canonical emphasises, winking to phone makers, that "if you already make handsets that run Android, the work needed to adopt Ubuntu will be trivial".
The user interface is interesting (video), and there's clearly some similarity to Unity here. A short swipe in the from the left, and you get a dock with icons of your favourite applications. A full swipe in from the left reveals a list of running applications similar to what you see on the N9/MeeGo. A swipe in from the right always brings you back to the last used application. Swipe in from the bottom reveals the application's own controls. A swipe from the top reveals a webOS "Just type"-like search tool. All of these edge gestures work straight from the lock screen, too.
What's really cool is that Ubuntu for phones will include the full desktop Ubuntu experience as well. You can dock your phone and it'll switch seamlessly to regular Ubuntu and Unity, including all the desktop applications you're used to. This is basically what Microsoft should have done, but didn't (i.e., Metro on phones/tablets, traditional desktop when docked).
All in all, the interface looks stunning. I can't comment on using it, of course, but it seems well thought-out, borrowing good ideas where it makes sense, while still looking distinctly Ubuntu. The full desktop integration is a major feature in my book - I really want to get my hands on this right now.
There's also some bad news. While it will be released and developed as open source (i.e., not behind closed doors like Android), carriers and device makers do get the ability to "enhance" (as Canonical puts it diplomatically) the operating system - and we all know what that means. There's no hardware partners lined up right now, but Canonical still states phones will be available early 2014.
Developers can get started here, so have fun.