This is NOT the official Billboard app (in fact, there's no official Billboard or Top 100 Android app yet).
Billy lets you browse the top pop, rock, dance and more genres right now, and listen to them. You can as well watch the official YouTube video of each of the songs and look at the best albums of artists.
Genres available:
Hot100 (Billboard's most popular chart)
Pop
Rock
Dance
Metal
Country
R&B
The songs list is provided by Billboard, who make and update industry-standard songs popularity charts like Billboard Hot 100 and more. Billy uses SoundCloud as source for streaming songs.
Billy is the quickest way to get to the top songs. Try it out!
If you are an Android developer, you might be interested in the source code. It's all open. https://github.com/vibinr/Billy