Population refers to the total number of people living in an area. Migration is defined as a permanent change in residence. Push and pull factors influence migration patterns - push factors motivate people to leave an area due to things like natural disasters, while pull factors attract people to new destinations such as jobs or opportunities. The rapid growth of the world's population in the past century has been attributed to increasing birth rates, decreasing infant mortality rates, and rising life expectancies. Population pyramids graphically display a population's age and gender composition using bar graphs to show the numbers or proportions of males and females in each age group, reflecting population growth or decline due to factors like immigration, birth, emigration, and death.