A control tower is a rather tall building located on the grounds of an airport. From this tower, air traffic controllers monitor and direct the movement of incoming and outgoing air traffic. There is at least one control tower at every airport with regular flights. These towers answer to one of the 21 air traffic command centers located around the U.S. Other countries have similar procedures.
When planes were first invented, there wasn't enough traffic to necessitate much organization. Most often, plane-to-land radio communications were only for emergencies or experimentation and people came and went as they please. After several scandals involving the post office and inexperienced pilots, the United States government pushed the combination of national weather services and radio stations to produce safer, more efficient travel. By 1934 the first control tower was built in Cleveland.
Since then, control towers have become a vital part of air travel, orchestrating the complicated routes of thousands of planes every day.
Some information from http://www.ama500.jccbi.gov/afss/History/FAA.htm and howstuffworks.com