The shape of a hemisphere is half a sphere, which is a three-dimensional circle. The word "hemisphere" is derived from the Greek and means "half a sphere." The Earth is often divided into four hemispheres: Northern, Southern, Western and Eastern.
The Prime Meridian and International Date Line divides the Earth longitudinally into the Eastern and Western hemispheres. The equator, at 0 degrees latitude, divides the planet into the Northern and Southern hemispheres. The northern and Southern hemispheres typically experience different temperatures due to the Earth's tilt during the different seasons.
The brain is also divided into two hemispheres: right and left. Each half has its own set of specialized functions.