Catholics are a type of Christian. They have a dude that they call the Pope who often wears a big hat and sits in a big chair sometimes. He's really important to Catholics. Other types of Christians don't have a dude called the Pope.
Catholics also have a system of officially sanctioned Saints (you can count them), whereas the designation of "Saint" in wider Christianity is more general (you can't really count them, because they are so numerous). There are other differences, but these ones are pretty major. The Pope thing being the main distinction.
The Catholic Church is a major political force that used to cause a lot of ruckus back in the day. Christianity is an all-inclusive belief system that simply requires you to follow the teachings of Christ. Popes in ye olden days were major political figures, now the role is more like the Queen of England or something. Like, if they say or do something, it makes headlines, but typically no one is going to go to war over it.
Thank you for asking.