Lesson 1 - Understanding light - Exposure, ISO sensitivity
Image taken from http://photoinsomnia.com/photography/shoot-bracketed-for-better-landscapes/
The picture on the left is underexposed, the one on the right is overexposed and the correct exposure is in the middle. As you can see there is a red number on the shots to the sides indicating how much do they differ - in our case - two stops from the neutral.
Another thing that is closely linked to exposure is ISO sensitivity. No, it's not a spy code but even I don't know what the ISO abbreviation stands for. In plain English ISO sensitivity stands for the sensitivity of the sensor to light. Those who have used film know that films have limited or fixed ISO (say, one or two numbers). Well - good news - cameras have many.
The rule is that THE HIGHER THE ISO THE LIGHTER IT GETS - IF ALL THE OTHER SETTINGS ARE THE SAME. So the higher ISO is determined by the bigger number - some cameras have options to have ISO of 12000 and even more.