AOSP is a term you'll see used a lot -- here, as well as at other Android-centric sites on the Internet. I'll admit I'm guilty of using it and just expecting everyone to know what I'm talking about, and I shouldn't. To rectify that, at least a little bit, I'll try to explain what the AOSP is now so we're all on the same page.
For some of us -- the nerdly types who build software -- the full name tells us what we need to know. AOSP stands for Android Open Source Project. The AOSP was designed and written by folks who had a vision that the world needed an open-source platform that exists for developers to easily build mobile applications. It wasn't designed to beat any other platform in market share, or to fight for user freedom from tyrannical CEOs -- it exists as a delivery mechanism for mobile apps -- like Google's mobile apps, or any of the 400,000+ in the Google Play store. Luckily, Google realized that using open-source software would ensure that this operating system/mobile application content delivery system is available for all, for free. And by choosing the licensing they did, it's also attractive to device manufacturers who can use it as a base to build their own mobile OS.
The premise plays out rather nicely. Google writes and maintains a tree of all the Android source code -- the AOSP. It's made available for everyone (you, me, manufacturers you've never heard of and not just big players like Samsung or HTC) to download, modify, and take ownership of. This means the folks at CyanogenMod can add cool stuff like audio profiles. It also means folks like HTC can change multitasking in ways that many of us don't like. You can't have one without having the other. The big players then use their modified version of this source to build their own operating system. Some, like Amazon, radically changed everything without a care to use Google's official applications and keep their device in compliance with Android guidelines. Some, like HTC radically changed everything yet followed the Android Compatibility Program (ACP) so they could include Google's core application suite -- including the Google Play store. Some, like the folks at CyanogenMod, enhance the pure AOSP code with additions but don't change the overall look and feel. Again -- that's how this open-source thing works. You can't have it without allowing folks to change it as they see fit, for better or worse.
Any of us can download and build the AOSP. We can even stay compliant with the ACP and contact Google about including their applications. Yes, any of us could build our own device using the AOSP code in our garage or basement with Google's full blessing. That's the beauty of the AOSP, and we wouldn't want it any other way.