Plant cells are usually rectangular, because of their stiff cell walls. Animal cells are usually rounded, because they don't have cell walls.

Most animal cells are not actually round, because they're interacting with all the cells around them, so they can be flattened or have extensions reaching out. Nerve cells reach out to each other with pointy extensions. One type of brain cells is called 'astrocytes.' They are sort of star-shaped.

White blood cells are round but with bumps all over the surface. Red blood cells are round but flat, with little dish shapes in the center of each side. They're called 'bi-concave.'

Some plant cells are rounded. My former student Magdalena Bezanilla gave a talk yesterday at a cell biology conference in San Francisco. She showed movies of growing moss cells that were rounded at the tip where they were growing.