Marvel can refer to a few things, but the most well-known is likely Marvel Comics. Here's a breakdown:

• Marvel Comics: An American comic book publisher founded in 1939. They're famous for iconic superheroes like Spider-Man, Iron Man, Captain America, & Thor. They also publish stories featuring fantasy, sci-fi, and action themes.
• Marvel Cinematic Universe (MCU): A hugely successful film franchise based on Marvel Comics characters. The MCU began in 2008 with Iron Man and has expanded to include numerous interconnected movies and TV shows.