A Brief History:

Experiments with radar began in the late 1800’s, when Heinrich Hertz saw that metallic objects reflected radio waves.1 In the early 1900’s, Christian Hulsmeyer acquired a patent for his Telemobiloskop, also called a Remote Object Viewing Device. Ships used this device to avoid collisions in fog.2
For the next couple of decades, there was not a great deal of progress in the use of radar. Robert Page, who later became the Director of Research for the U.S. Naval Research Laboratory, demonstrated the pulse radar in 1936. Experiments, conducted by Page at the U.S. Naval Research Labs (together with Dr. Alfred Taylor and Dr. Leo C. Young) were able to detect basic continuous wave patterns made by ship and aircraft engines, by barely visible indications on an oscilloscope.3 By timing the pulses on an oscilloscope, the direction of the antenna revealed the angular location of the targets and equally important, range could be determined. Together, the target was located… a “fix” was made – relative to where the antenna was located. As a result of this work, Page, Taylor, and Young have been credited with authenticating the world’s first radar.4
The rapid development and success of modern radar during WWII cannot be understated… it was crucial to ending the war in the Allies favor and sooner rather than later. In 1939, the United States Navy came up with the acronym, RADAR, meaning “RAdio Detection And Ranging.”
A major development in the use of radar happened when physicists, John Randall and Harry Boot from the United Kingdom, invented the cavity magnetron5,6 (also in 1939) a device that shortened the pulse of radio wave energy and allowed for smaller radar systems as a whole. By the end of the war, a wide variety of land and sea-based radars existed… and the use of radar for civilian purposes became an exciting proposition. Smaller systems allowed for greater mobility and use over numerous platforms; all of this made radar systems more efficient and more accurate.
When WWII ended, scientists and inventors focused on peaceful uses for radar. To these fields, the use of radar was a new and exciting concept. For civil aviation, radar was of obvious value, but for fields including medicine, meteorology, and marine navigation, the potential use of radar seemed endless. At the same time, the radar gun used by police officers across the country, began to catch drivers with massive V–8 engines that went much faster than the sixty to seventy mile per hour posted limits of the 1950’s and 1960’s. For offenders, early on, this unfamiliar device must have added a great deal of aggravation to the whole ordeal.
Soon, radar became a household word, and people wanted to know not only the ways in which radar could make life better, but how this new technology worked. Initially, the public had concerns about whether these “waves” were safe. Later, convinced they must be, society called for easier meals.
An Introduction to Radar Technology:
Most people have heard of the term “radar waves” and are familiar with the idea that radio waves use radar to somehow “detect an object” that is out in the distance or out in an area somewhere. Fans of submarine movies know that sonar (SOund Navigation and Ranging) uses sound to either communicate with or detect other ships or submarines on or under the surface of water. Using the distance it takes for that sound to reach the object (and some reasonable mathematical calculations) the radio operator could report to the captain not only the distance of other ships, but also their course, speed, and depth – after several measurements were taken.
Today, for the most part, a majority of people accept the fact that there are all kinds of waves out there, radio and microwaves included, bouncing all around us – indeed through us… and these waves make all sorts of things work. Microwave ovens, cell phones, Wi-Fi, cat scans, x-rays, and numerous other devices – use invisible (to the naked eye) waves to somehow “just work”. And as long as these waves don’t cause harm, (and many will always question that) and the devices work, the specifics of how they function are not necessary. Nonetheless, a basic comprehension of radar is possible without flashbacks to high school physics!
What makes radar technology difficult for many people to understand, is that – although radio waves and microwaves (two types of electromagnetic waves) are very small and are, for the most part, not seen, they do transport energy – just like ocean waves and sound waves. Ocean and sound waves require matter in order to transport energy, while electromagnetic waves do not. Think of the enormous power of the sun; while ultraviolet rays (another form of electromagnetic wave) carry no matter, they are capable of severe burns from 93 million miles away in a very short time! Further, radio waves can pass through vacuums, which allows them to travel through space – making the use of satellites possible. All electromagnetic waves travel at the speed of light (or so close to it, that any difference is inconsequential in virtually any application), a fixed number equal to 299,792,458 meters per second (take a moment to think about how really fast that is… in feet if need be… 1 meter = 3.28 feet!). This speed, almost always referred to in science as the letter “c” for example, E = mc2… Albert Einstein’s theory of special relativity, shows that increased mass, “m” of a body comes from the energy of motion of that body, i.e., the kinetic energy, “E” divided by the speed of light squared (c2). That sounds fairly simple and straightforward, but Albert Einstein is still many generations ahead of his time!
Although some aspects of radar technology are very complex, involving higher mathematics, the basics of how radar works is fairly straightforward. A radar system has a transmitter that sends out radio waves called radar signals in a direction determined by the radio operator7. These radar signals (made up of radio waves and microwaves – types of electromagnetic waves) bounce back to a transmitter for interpretation. As seen in Image A, below, the basic concept of a sending out radio waves from a transmitter/sender over a distance “r”, allowing them to hit an object, having the reflected wave (shown in green) travel back over the same distance, “r”, back to the receiver – is a simple idea. Using high school mathematics, one can see that measuring the time it takes for the wave to travel from the transmitter to the object and back , with a known radio wave speed – determining the object’s distance is a simple calculation.

Sources:

1 http://www.en.wikipedia.org/wiki/Heinrich_Hertz
2 http://www.en.wikipedia.org/wiki/History_of_radar
3 A typical definition of an oscilloscope is: “An oscilloscope is a laboratory instrument commonly used to display and analyze the waveform of electronic signals. In effect, the device draws a graph of the instantaneous signal voltage as a function of time.” It allows observation of a constantly varying signal over time. For example see:
http://www.whatis.techtarget.com/definition/oscilloscope

4 Raymond C. Watson, Jr., “Radar Origins Worldwide”
Trafford Publishing, 2009, p. 45., cited by http://en.wikipedia.org/wiki/History_of_radar

5 http://www.ieeeghn.org/wiki/index.php/Cavity_Magnetron
6 Magnetrons are still in use today; they are found in microwave ovens and some radars still contain them. See:
http://en.wikipedia.org/wiki/Microwave_oven#Cavity_magnetron

7 Image located at: http://www.en.wikipedia.org/wiki/Sonar#mediaviewer/File:Sonar_ Principle_EN.svg