A hemisphere is half of the Earth. The Earth can actually be divided into hemispheres in two ways: by the equator, and by the Prime Meridian (through Greenwich, England) at 0 degrees longitude and another meridian at 180 degrees longitude (near the location of the International Dateline in the western Pacific Ocean). The equator divides the Earth into northern and southern hemispheres. There are seasonal differences between the northern and southern hemispheres but there is no such difference between the eastern and western hemispheres. Zero and 180 degrees longitude divide the Earth into the eastern (most of Europe, Africa, Australia, and Asia) and western (the Americas) hemispheres.
The Earth is not a perfect sphere but rather an ellipsoid. As the Earth spins, centrifugal forces cause the planet to bulge slightly around the middle.