To understand 'V For Vendetta', you have to understand the times in which it was written. Britain of the 1980s was wracked by recession and with discontent against the Conservative government. Hanging over this was the ever-present Cold War mentality.
The premise of this book is that the Conservative government are voted out (as Moore expected at the time, but which didn't come to pass) and the new Labour government insists on nuclear disarmament. So it is that when the Cold War becomes World War 3, Britain is uninvolved, but not unaffected.
As nuclear fallout affects the weather and radiation causes widespread death a fascist regime rises to offer England stability and order. The price for this is the internment and execution of blacks, homosexuals, liberals and all those who don't conform to the fascist ideals. The future Moore paints is a bleak and painfully believable one.
However, one man decides to destroy this new order so that freedom can be rebuilt from the rubble. The man is known only as V. When it comes down to it, V is the greatest element of this book. He's cultured, witty, mysterious, charismatic and ruthless. Also, David Lloyd's design of the character as a man dressed in a smiling mask and Guy Fawkes costume is inspired. My favourite bit of the book is when V sneaks into the home of a child-abusing bishop and confronts the corrupt clergyman with a Rolling Stones quote; 'Please allow me to introduce myself, I'm a man of wealth and taste'.
There are numerous other characters integral to the story; the widow Rose, forced to become a stripper; the Leader, an insane fascist in love with Fate, the computer which effectively controls England; as well as the various heads of the Party. However, there are two characters who reveal about V what he never reveals himself.
The first is Mr. Finch, a police officer tasked with hunting down V, who undertakes a personal quest to understand V's mind (which includes an LSD trip in a death camp).
The other character is Evey. At the beginning she is a helpless girl who turns to prostitution to make ends meet. However, after meeting V she begins an education at his hands in the meaning of freedom which breaks her and then remakes her.
At it's most basic level this book is about freedom, both personal freedom and freedom as an abstract concept. Dark, disturbing and thought-provoking, this book still manages to be exciting and uplifting. Certainly one of the best pieces of literature (not just comic-form) that I've ever read.