The Earth is rounded because of the gravity acting on it while it is spinning around (rotating).
The Earth is rounded in shape because of the gravity that applies on the planet as it spins. The gravity pulls the mass of the Earth in towards the center, which ends up creating the most efficient shape, a sphere. While the force of gravity on small objects is tiny, it tends to get stronger as the object gets larger. Which is why, asteroids tend to keep their shape. Their central gravity is not strong enough to overcome the ability of the rock to keep its shape. However, in objects larger than about 1,000 km in size, the central gravity is strong enough to pull the object into the shape of a sphere.
In fact, in 2006, the International Astronomical Union (IAU) decided that in order to be called a planet, the object must orbit the Sun, need to have cleared out all the smaller objects in its orbit, and must need to have enough gravity to pull itself into a sphere. When an object has the gravity to pull itself into a sphere, it is said that it is in hydrostatic equilibrium. Plus when the planet is forming, it is hot and melted, which aids gravity in pulling it into a spherical shape.
Still, Earth is technically not a perfect sphere. In fact, it is an oblate spheroid, a bit flatter on the poles and with a slight bulge in the center. So, even though the Earth appears to be round, it is not. This mainly occurs because of the Earth’s axis. As the planet tilts on its axis at least once every 24 hours, it causes its equator to bulge outwards.
According to geologist Vic Baker at the University of Arizona in Tucson, the Earth has "a bit of plasticity that allows the shape to deform very slightly. The effect would be similar to spinning a bit of Silly Putty, though Earth's plasticity is much, much less than that of the silicone plastic clay so familiar to children."
Furthermore the mass on the planet is distributed unevenly. A greater concentration of mass tends to cause bumps on the surface. This concentration of mass is constantly changing due to a variety of factors, such as shifting magma, plate tectonics, and even meteors hitting the surface. All these factors further help change the mass concentration on Earth, which restrict the planet from becoming a perfect sphere.