QML is the name of the language (like C ++, which is another language ...)
QtQuick is a toolkit for QML that allows you to develop a graphical interface in QML (there are other tools for QML, some of them are graphical, for example Sailfish Silica or BlackBerry Cascade , and some are not graphical, for example QBS , which is a replacement for QMake / CMake / make ...)
QtQuick 1.x was based on Qt4.x and used the QPainter / QGraphicsView API to draw the scene. QtQuick 2.X was introduced with Qt5.0 based on Scene Graph, a highly optimized OpenGLES2 abstraction layer.
With Qt5.1, the scene graph has been expanded to use multi-threading (QtQuick 2.1) With Qt5.2, the Scene Graph is still much optimized to reduce CPU / GPU calls and memory usage.
The QML mechanism was based on JsCore (Webkit JS engine) in Qt4.x and was reinstalled on V8 (Google Chrome JS engine) with 5.0, but this prohibits its use on mobile phones, especially on iOS, so Qt5.2 introduced a new QML engine named V4VM, created by the guys / for Qt.
There are also QtQuick Controls, which are a set of widgets based on QtQuick. It was originally intended for desktop computers, but Qt 5.4 introduced its own L&F for Android, based on a bare theme. The material theme as well as the iOS theme are under development, but are not available in the current version of Qt (5.5). Some controls were Enterprise only, but in Qt5.5 they were renamed Extras and are now available for all licenses. Another development is called QtQuickControls 2, which completely rewrites the controls in order to get the best performance, aimed at lightweight built-in interfaces, it should be at the preview stage in Qt 5.6.
From Qt5.5, a new module appeared, called QtQuick3D, which makes it possible to create 3D applications / games using the QML language. It does not use SceneGraph, which is too 2D / 2.4D oriented. For this use, the new engine is called FrameGraph.
If you are developing modern applications, you should use Qt5.x + QML 2.x + QtQuick 2.x to touch the most extensive user base.
In Qt, as a rule, an update always follows, because they add more features, more features and platforms.