In the early days of the space program, (can you remember back that far) Mariner 10 was on it's way to Venus and then on to Mercury. In those early days spaceflight was new and people had to be inventive with the equipment available. After the launch the spacecraft was being tested to make sure that everything was operating correctly when a problem threatened the mission. An oscillation occurred between the gyro stabilizing system, the attitude control jets, and the solar panels. The spacecraft was consuming it's attitude control fuel at an alarming rate and the mission was in danger. Several steps were taken to regain control of the spacecraft, but the ultimate solution was found inadvertently. As Mariner drew closer to the Sun, the solar panels were turned away from the Sun to keep them from over heating. It was suddenly discovered that the angled solar panels were in fact correcting the attitude control problems of the spacecraft. Mariner completed it's mission to Mercury using solar sailing as it's primary attitude control system.
In solar sailing the force of light is used to accelerate a spacecraft. The method was first proposed in 1946. A large reflective mirror is the sail and is angled to the Sun to control the direction of the force. Now the first thing you have to understand is that we're not talking about a lot of force here, so the sail has to be really big, like miles square, and it takes a long long time to get anywhere, but the thrust is free once you get the sail rigged.
And now a little math, for each square meter of sail:
Acceleration = 1400 / ( Au squared ) * C * ( grams / square meter of sail )
Acceleration is in meters per second squared
Au is the distance from the Sun in astronomical units ( at Earth =1 )
C is the speed of light 300,000 Km/sec
At Earth a 1000 sq meter sail massing 5kg will have an acceleration of 0.00186 m/sec sq. At Mercury, the same sail would give 0.01674 m/sec sq, nine times as much, so you can see that solar sailing is an inner solar system game. Time of course is the element needed to build up high velocities. For that reason solar sailing is good for robotic craft hauling cargo, but not for humans who need to travel through space quickly.