Linear capture

The camera sensor responds to light in a different way to our eyes and to film.  Linear is the way in which the camera reacts to light falling on it.  Our eyes and film compress the way we receive light so what may seem twice as bright is less so in our vision.  Our eyes can handle a wider range of brightness than the camera's sensor.

Although when we take a picture it may look like we would expect it to on our LCD display screen it is not the way in which the sensor has captured the image.  The camera processes the image for us and adjusts it so the image looks brighter when we get to see it.  If this processing didn't take place we would be left with a rather dark image.

If we shoot Raw we can get an unprocessed image from the camera and then process it ourselves using Photoshop of other image processing software.  Shooting raw gives you more control over your final image.

Gamma
The correction that the camera makes to your image image is called a gamma corrections curve which makes the shot look normal in appearance.

Example of a gamma correction curve.

What is gamma?

It's the term used in computing and to describe digital images.  When applied to monitor screens it is a measure of the relationship between voltage input and the brightness intensity. Because of the way in which computer monitors work a raw, uncorrected photo would look darker and more contrasty than our eyes would expect.

In compensating for this, gamma correction is applied by the camera post capture.  This usually makes the image brighter and less contrasty as it lifts dark tones more than light ones. 

In Lightroom you can adjust the tones using Tone Curve.  This helpfully highlights the area of the image that is being affected by the adjustment you are making to the curve.  

In this example, the word Lights is highlighted to show you that the area on the image that is being adjusted affects the lights in the image.