Judo is a form of grappling that is mostly composed of throws and trips with the use of the "gi" (the uniform that you see them wearing). It doesn't compose of any striking (punches, kicks), just pure grappling. There is something called "newaza" in which they go to the ground and wrestle each other, but it's not the main point of judo. When they go to the ground, under certain circumstances they are able to continue wrestling on the ground, but a lot of the times will be stood back up by the referee. This is why if you look up Judo matches, they mostly take place standing.

Just to add on, I'll list some other common martial arts you might have heard of

• Karate aka the "Dane Cook" of martial arts - Karate has a lot of different types, so it's kind of hard to say definitively what it is, but here is the gist. Karate is a Japanese martial art that is almost purely of striking. Punches, knees, kicks, and elbows are all allowed, but the only strikes allowed to the head are kicks (which is kind of weird IMO). The image of karate in the mainstream media seems to be a bunch of little kids just screaming "hyah!" while throwing weak punches. They are geared from head to to, and exist purely for light-hearted sportive fun. What they don't realize is that there is a pretty wide range of karate, and they are rarely exposed to what is called "Kyokushin karate". It's fucking brutal. It's bare knuckled, close proximity, and full contact. It's kind of old fashioned mindset of "being a man", they stand in front of each other and just beat each other until the other one falls. So in conclusion, even if someone says that they do karate, you can't determine for sure what kind of practitioner they are because of the wide variety of it.

• Tae Kwon Do - This is a Korean based martial art that has unfortunately developed a very bad reputation in the martial arts world. This is due in part to it's very sportive nature and it's kind of deviation from traditional values (they give out black belt like candy to little kids). This is a true story. I took korean class during my high school career, and there were literally 4 black belts in my class. This is my sophmore year. This is unfortunate as there are good practitioners out there, but they are often disregarded when when they say that they are taking tae kwon do. It's also a striking martial art (although some schools teach joint locks and throws) that is almost exclusively kicking. When you see a Tae Kwon Do match, it will be almost be entirely composed of kicks (which are often criticized for being really flashy and impractical at times).

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Edit: Replied to the wrong comment gah! I meant to reply to lethargicwalrus.