This article from Luis de la Rosa at savvy apps discusses "The State of Android Emulators." We've posted a few different options for Android emulators in the past - VirtualBox emulation, Intel's HAXM, and Genymotion, for example - all of which are mentioned in de la Rosa's article. However, rather than focusing on the details of any particular emulator, de la Rosa explores all of the options together and discusses their origins and uses.
It's not an extremely detailed or particularly critical overview - de la Rosa presents all of the emulators in a fairly favorable light - but it is a great summary of the current state of things as far as Android emulators go, compiling in one place most of the popular options and reasons for choosing one as opposed to simply working off a physical device.
Check out the full article if you're looking for some ideas on which Android emulator to choose, or just want a better idea of what's out there.