Unclear about what Android apps run on

[ala6]

This is a real newbie question, but i can't find an answer anywhere: What are all the existing Adroid apps running on? I know about the T-Mobile phone, but prior to that, were the apps being run on cell phones that had the runtimes installed, or were they just running on the platform the developer downloaded the SDK to?

[Buck]

Android emulators are/were used to simulate the Android platform. Developers for the Android challenge had to do the best they could in developing for the Android simulated environment.

There's been various blog posts showing actual handsets running ports of the Android platform, but i doubt they were ever stable enough to run applications on, most of them were only capable of launching a web browser etc.