http://www.katyperry.com/
Look at the above for some photos, video and songs.
I think her first single is the most recognisable, it's called "I kissed a girl (and I liked it)".
She is also famous for (now) being Mrs Russell Brand.
And regarding my comments above, don't get me wrong, I don't actually think Katy Perry is bad singer. I just think she's good rather than great.