The Earth is round, like a ball, but it is also slightly squashed. We say that its shape is roughly spherical.
The Earth travels around the Sun once every year.
The Earth also spins on its own axis. The axis is an imaginary line through the centre of the Earth from the North Pole to the South Pole. The Earth spins round once every 24 hours. This causes day and night on Earth.
It is day on the part of the Earth that is facing the Sun.
It is night on the part of the Earth that is facing away from the Sun.