Planets are round, or very close to it: their overall shape is a sphere. According to NASA, this happens because of gravity. Once a forming planet gathers enough mass, its gravity pulls equally from the center in every direction, which squeezes the material into a ball.
Not every planet is a perfect sphere, though. Mercury and Venus are the roundest planets in our solar system and are nearly perfect spheres. Fast-spinning giants bulge outward at the equator, a feature called the equatorial bulge: NASA reports that Saturn is about 10.7% wider around its middle than from pole to pole, and Jupiter is about 6.9% wider. Scientists call this slightly squashed shape an oblate spheroid.
Source: NASA Space Place, Why Are Planets Round?
Image: Saturn during equinox, NASA/JPL/Space Science Institute, public domain, via Wikimedia Commons.