Pls. refer here, "Install Android SDK on Eclipse 3.5 Galileo", for update information. ~edited 2009-07-18.
One of the development platform, which I used, is (JDK 6 + Eclipse + Android SDK) run under WindowsXP.
JDK 6 is the Java Development Kit from Sun.
Android SDK is the core to develope Android application.
Eclipse IDE for Java Developers is a free Open Source IDE for Java development. Android Development Tools plugin is needed, in order to work with Android SDK.