AOSP is the term that is going to be popular in the future. AOSP stands for Android Open Source Project. It is a project which is designed for developers who needed an open-source platform so that they can easily build mobile applications. Its objective is not to beat any other platform in the market; instead it is intended to be used as a delivery mechanism for mobile apps like Google’s mobile apps. It is also free and with the correct licensing, the device manufacturers can use it as a base to build their own mobile OS.
Samsung Galaxy S III Mini and Samsung Galaxy Nexus are among the first wave of smartphones which received the latest rendition of Google’s OS. Since it is the early build, bear in mind that there will be a lot of issues with the Google Apps. Suite even the fundamental features such as audio and camera is no working. These issues will be fixed in a timely fashion.
For Samsung Galaxy S III mini will be showcased one of the first AOSP Lollipop ROM. This will prove to Samsung that even older devices can run the newest builds flawlessly. You can try S3 Mini Lollipop development thread to learn more.
Of course S3 Mini will not be the only device that received a working Android 5.0 build. Two of the Galaxy Nexus will be also be using this build. This ROM is at a beta stage so there are a lot of things which is not working properly.