Linux-based Operating System running a Dalvik Virtual Machine produced by Google, typically seen in portable devices such as cellphones, e-book readers, tablets, etc. Not to be confused with Androids in the robotics sense.
Android is an linux based operating-system running a Dalvik Virtual Machine (Similar to java, and indeed based partially on) produced by Google, typically seen in portable devices such as cellphones, e-book readers, tablets, etc.
Android based devices are popularly used in hobbyist embedded projects, as they are fully functional computers often with multiple easy to interface wireless connections, easy to program for, cheap, and widely popular.
The term Android is a synonym for Robot with a Human(oid) appearance. This can sometimes be generically referred to as a Droid (though this is a Lucas Arts (Of Star Wars fame) registered trademark). Droid is also a line of Android cell phones produced by Motorola.