Up Up Down Down Left Right Left Right B A: the code has appeared in 100 video games and featured in TV shows throughout popular culture. It was first revealed in issue 1 of Nintendo Power.
But why was it created in the first place?
A Konami employee needed code to complete testing of a game, Gradius, because it was too hard. The code was left in when released on NES. No one knows if it was left in by accident or intentionally. The code was then used by others for testing.
It was most famously used in Contra to give you 3 lives from 30 lives.
It was then revealed in the first issue of Nintendo Power.
A few other popular games that it was in:
Ninja Turles - 2 extra continues.
Tengen Tetris - it drops a long tetris block
Castlevania Bloodlines - it unlocks Expert Mode.
For a more in depth look at the Konami Code, check out The Gaming Historian channel on Youtube.