The equator is an imaginary line dividing the earth into North and South Hemispheres, at 0° latitude. The prime meridian is an imaginary line dividing the earth into East and West Hemispheres, at 0° longitude.
In general, equators are features of all orbiting planets, defined as "the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and midway between the poles" (Wikipedia ). The Equator on earth is an interesting place because of the Earth's rotation. The difference between seasons is minimal in areas near the Equator, and temperatures are generally high year round (with exceptions based on altitude).
The prime meridian
Unlike the equator, which is determined by the Earth's axis of rotation and the location of it's poles, the prime meridian is essentially arbitrarily drawn. It's definition has changed over time in different parts of the world. Today's definition, the "Greenwich Meridian" was selected in 1884, by the International Meridian Conference in the United States, which also determined the standard of time zones used throughout the world.
Everything west of the prime meridian is called the western hemisphere, and east of it is the eastern hemisphere. At the opposite end of the Earth, at #180^0# longitude is the meridian where east and west also meet.
Sources: Wikipedia 1 2