What Is Android?
On November 5th, 2007 leading technology and wireless companies came together to announce the future development of a truly open platform for all kinds of mobile devices - Android. Leading this development are Google Inc, T-Mobile, Intel, HTC, Qualcomm, Motorola along with many other companies under the umbrella of the Open Handset Alliance - a global alliance between technology and mobile industry leaders.
http://www.talkandroid.com/google-android-faq/
There are two steps to develop android application
step 1: Download eclipse
http://www.eclipse.org/downloads/
step 2: Install SDK
http://developer.android.com/sdk/1.1_r1/installing.html
Developer guide
http://developer.android.com/guide/index.html
1. How to work with eclipse
http://developer.android.com/guide/developing/eclipse-adt.html
2. In Other IDEs
http://developer.android.com/guide/developing/other-ide.html