Population growth is the rate of increase in the number of people in a given area, such as a city, country or continent. Population growth is closely linked to fertility, which is the rate at which women produce offspring. Due to the limited resources on earth and in various countries, rampant population growth can lead to inadequate food and other necessities, so in certain cases it can be beneficial to attempt to limit population growth.
One way for a country or other ruling authoritative body to control population growth is to institute laws that prohibit high fertility. China, for instance, is well known for instituting a "one child policy" which makes it illegal for urban couples to have more than one child. Using regulations to control population in this manner is often frowned upon, and requires a governing body with supreme authority over its people, but it can be very effective.
Another important way to limit population growth is to increase the rate of contraception among the population. Contraception, while not infallible, drastically reduces fertility rates, often with minimal expense. While contraception and education about using contraception is widely available in rich countries, people in poor countries may not have access to simple contraceptive methods like condoms, or even know what they are. Not only can poor nations in areas like Africa and southeast Asia benefit from the fertility control aspect of contraception, but condoms can also help prevent the spread of STDs like AIDS, which is an epidemic in many poor nations.
Economics plays a vital role in determining population growth. Countries that are well developed tend to have lower population growths and are more urbanized, with a higher cost of living, while poor countries are more rural with a lower cost of living. When the cost of having a child is extremely high, it deters parents from having more than a couple of kids. When the costs of raising children are low, such as they are in rural areas where they may contribute greatly to work on a farm, families have a much greater incentive to have babies. Encouraging economic development and urbanization in less developed countries may eventually lead to lower population growth.
While not designed for population control, there are several other factors which tend to limit population growth. Wars are one factor which plays a large role in limiting population growth. Since soldiers are typically able bodied men, any combat deaths are likely to decrease population growth. Controlling population is a very controversial subject, and many consider education and making contraception available, but not forcing its use to be the only acceptable way to promote population control. Even if birth rates can be controlled to a mere replacement rate (every couple has two kids, replacing the mother and father) increasing sophistication of health care has led to an increase in life expectancies, which has further increased the size of world populations.