It's about a world filled with political intrigue and power struggles. The story mainly follows Lelouch, who obtains the power of Geass, which allows him to command others to do his bidding. With this power, he sets out on a journey to change the world that is under the control of the Britannian Empire, aiming to create a better place for his sister and all those who are oppressed.
Yes, it is. As a 'Code Geass' fan, it gives you more content related to the series. You get to explore the story further and might discover new aspects of the characters and the world.
Code Geass did start as a manga. However, its anime version became even more widely known and beloved. The manga provided the foundation for the rich story and characters that were further developed in the anime.
Code Geass started as an anime. The manga was based on the anime's success and came after to expand the story for fans. Some elements might differ between the two, but the anime was the original form.