Bluestacks App Player is a compatibility layer and a emulator that enables a Windows PC install and execute Android programs and integrate them on Windows Desktop without need of the Android SDK or a virtual machine.
Basically the Bluestacks share the same architecture of Wine, in this case run Android apps without Android in the same way that Wine enable to run Windows applications without Windows.
Since Bluestacks is an alternative Android subsystem (Wine's best definition is also that), acess to official Android Market, core Goggle programs that smartphones and tablets have, device drivers, codecs will not work. (Like Wine, Bluestacks implements the Android user mode, not the kernel mode that includes a modified Linux kernel and device drivers).
The original Android kernel mode in Bluestacks environment are replaced by the native Windows subsystems (including audio/video codecs, filesystem, security, touchscreen handling, etc) which mimics the same behaviour. (like Wine does. In fact, the Bluestacks fake Dalvik VM acts like the Wine's wineserver)
Even in this early stage, Bluestacks can use some alternative Android market (like Amazon) and sync some Android apps between an real Android device and a Windows computer.
However the best way to install applications is to manually download the APK package from the Internet (some freewares have an official open webpage to download legally) and install using the built-in package manager to that.
Some features will be implemented in future versions, like an ARM emulator to run some applications that have native ARM code packaged in some subrotines.