Code Geass is like a cross between Gundam Seed, Death Note, and V for Vendetta.
This description isn't perfectly accurate in every way, but it conveys alot about the series in one succinct statement, which is useful if you're introducing the show to someone new.
Let me summarize the components of the comparisons.
From Gundam Seed: Mecha. Childhood friends on opposite sides. Kira -> Suzaku. Lacus -> Euphie. Cagalli -> Kallen. Athrun -> Lelouch, but different. That's where Death Note comes in...
From Death Note: The premise; A disgruntled student gains power to change the world like he always dreamed of, and goes about it in anti-hero fashion. Light -> Lelouch. Ryuk -> C.C.
From V for Vendetta: A charismatic terrorist in cape and mask fights an oppressive British regime. He employs dramatic exploits and publicity stunts to sway the people to his cause.