User Interface:
http://developer.android.com/guide/topics/ui/index.html

Resources and Assests:http://developer.android.com/guide/topics/resources/index.html

Intent and Intent Filters:http://developer.android.com/guide/topics/intents/intents-filters.html

Data Storage:
http://developer.android.com/guide/topics/data/data-storage.html

Content Providers:http://developer.android.com/guide/topics/providers/content-providers.html

Security and Permissions:http://developer.android.com/guide/topics/security/security.html

Graphics (2D/3D): http://developer.android.com/guide/topics/graphics/index.html

Audio and Video:http://developer.android.com/guide/topics/media/index.html

Location and Maps:http://developer.android.com/guide/topics/media/index.html

App Widgets:http://developer.android.com/guide/topics/appwidgets/index.html

Signing Your Applications:http://developer.android.com/guide/publishing/app-signing.html

Versioning Your Applications:http://developer.android.com/guide/publishing/versioning.html

Preparing to Publish: A Checklist:http://developer.android.com/guide/publishing/preparing.html

Publish Applications:http://developer.android.com/guide/publishing/publishing.html