Lumia Camera was supposed to arrive with Lumia Denim update but while Lumia 830 comes packed with Lumia Denim firmware, it lacked Lumia Camera. Now, AT&T is going to announce Lumia 830 soon (most probably this week), so may be Lumia Camera comes pre-installed on AT&T Lumia 830 and others may get it soon.
Lumia Camera brings features like Moment capture, Rich capture, better imaging algorithms. Read all about Lumia Denim here and read all about Lumia Camera here.
Thanks Raj23 for the tip. Cheers!!