Seasons occur on Earth because the planet is tilted on its axis at just over 23 degrees. Both the Southern Hemisphere and the Northern Hemisphere are tilted closer to the sun as the planet completes a full orbit.
The Earth's tilt means that, as the planet revolves around the sun, first one pole and then the other is closer to the sun. At the most extreme points, parts of one pole are in complete darkness for long periods, while locations at or near the other pole are in perpetual daylight.
If the Earth had no tilt at all and rotated straight up and down on its axis, there would be no seasons as they are currently known. While there would still be an equator and poles, with all parts of the planet remaining the same distance from the sun, the seasons would end. Points close to the equator experience this to an extent. Locations close to the equator tend to have just two seasons: a dry season and a rainy season. Even though they are affected far less by the actual tilt of the planet, the seasons near the equator are still strongly influenced by the weather patterns in other parts of the world.