Microsoft knows the market for Android is big (same goes for iOS) and often releases applications or features for competing platform before its very own Windows Phone. OneNote Mobile however is an offering (a key one at that) on Windows Phone but the company considered it was best to release and Android client too (the iOS client is already out there).
You can now take notes on your Android-powered phone, including text, pictures, checkboxes and bullets and sync them using SkyDrive. You’ll be able to pick up work from your PC or web browser and continue on your Android smartphone. The app is available for free in the Android Market linked below. You’ll need Android 2.3 Gingerbread or a newer version to run it.
Source: Android Market