Our sister site, http://developer.android.com/, includes feature overviews of the various Android platform versions. The links below will take you to developer.android.com where you can view this information.
The links below will navigate you away from this site.
Android 2.3
Android 2.3 corresponded to the "Gingerbread" milestone branch, and has an API level of 9.
Android 2.2
Android 2.2 corresponded to the "FroYo" milestone branch, and has an API level of 8.
Android 2.1
Android 2.1 corresponded to the "Eclair" milestone branch, and has an API level of 7.
The Eclair branch was also used for 2.0 and 2.0.1; however, both of those releases were quickly obsoleted by the version 2.1 Eclair release. As Android 2.1 includes key bug fixes and improvements not present in 2.0/2.0.1, only Android 2.1 should be used for new devices. As there is no compatibility program for 2.0 or 2.0.1, the officially compatible Eclair-based release is Android 2.1. (The linked document refers to Android 2.0, because there were no new platform features added in 2.1.)
Android 1.6
Android 1.6 corresponded to the "Donut" milestone branch, and has an API level of 4.
Android 1.5
Android 1.5 corresponded to the "Cupcake" milestone branch, and has an API level of 3.
Android 1.1
Android 1.1 has an API level of 2. Android 1.1 was known as "Petit Four" internally, though this name was not used officially.
Android 1.0
was the first release of Android, and has an API level of 1. Since it was the first released version of Android, no platform highlights were prepared for this release.