Gravity is one major force that creates tides. In 1687, Sir Isaac Newton explained that ocean tides result from the gravitational attraction of the sun and moon on the oceans of the earth.
Water molecules are attracted to each other and tend to stick together. (This is the reason why two droplets of water merge quickly into one when they touch each other, and why a single drop of water will sit as a little ball on the table rather than spreading out into a thin film). This property of water is called "cohesion". Because water is cohesive, it tends to act together like a single body or mass. You can think of it kind of like a weak blob of Jello: when you push or pull on one part, the rest of it moves too.
As the earth rotates, it is subjected to a centrifugal force which pulls the ocean water molecules away from the earth's surface. (This is just like when you spin around very fast, and your arms want to fly away from your body).
Because the ocean water molecules stick together, this caused the entire ocean to bulge outward in the center, creating a kind of "mound" of water encircling the earth. Because of this, the world's oceans bulge around the equator, and get thinner toward the poles.