2.1 Eclair is outdated, rarely found on devices now.
2.2 Froyo is outdated, usually found on cheap tablets and phones that haven't been updated by the mfg.
2.3 Gingerbread refined the user interface, improved the soft keyboard and copy/paste features, improved gaming performance, added SIP support (VoIP calls), and added support for Near Field Communication.
3.0 Honeycombwas a tablet-oriented release which supports larger screen devices and introduces many new user interface features, and supports multi-core processors and hardware acceleration for graphics.
3.1 Honeycomb, released in May 2011, added support for extra input devices, USB host mode for transferring information directly from cameras and other devices, and the Google Movies and Books apps.
3.2 Honeycomb, released in July 2011, added optimization for a broader range of screen sizes, new "zoom-to-fill" screen compatibility mode, loading media files directly from SD card, and an extended screen support API.
4.0 Ice Cream Sandwich, announced on October 19, 2011, brought Honeycomb features to smartphones and added new features including facial recognition unlock, network data usage monitoring and control, unified social networking contacts, photography enhancements, offline email searching, and information sharing using NFC.