Most animal cells have a round shape. Unlike plants, animal cells do not have a hard wall structures to keep them in a certain shape. If you look at plant cells under a microscope, they look like bricks stacked together with straight walls. The plants need cells with hard walls to grow tall and not fall over. But if you look at animals, they look more round. Animals cells can be round because we have things like bones that help keep shape so the cells don't have to have hard walls.

The shape of a cell that has no cell wall (which animal cells do not) is basically whatever it happens to fill up, since a cell wall is just a layer of chemicals that don't like to break, like a bag. This means that cells can change shape, either due to running into stuff, or because the cell itself is trying to move.