Memes

Animals are one group or "kingdom" of life. Plants (such as trees) and fungi (such as mushrooms) each have their own kingdoms, and so do bacteria and a few other forms of life. They're organized this way to represent how closely related they are. Every single living thing in the animal kingdom is more closely related to every single other thing in the animal kingdom than to anything in any other kingdom.

As an example, chimpanzees, starfish, and earthworms are more closely related to each other than to a sunflower, so we call chimpanzees, starfish, and earthworms animals but not sunflowers. This is called "taxonomy" and there's a ton of different levels of how related things are, ranging from very distantly related to so closely related you can barely tell them apart. Kingdom isn't even the most broad!

You might have also heard that fungi are more closely related to animals than to plants, but that doesn't mean that fungi are animals, just that the lifeform that branched into fungi and animals did so a lot later than the one that branched into plants. In the end they're still distinct enough that we call them different kingdoms!