Digital cameras are set by default to lock focus and exposure together when the shutter release button is pressed halfway. Some digital cameras have a very useful feature to lock focus and exposure independently.
Besides Auto Focus Lock and Auto Exposure Lock, a camera may also have Flash Exposure Lock.
AF-Lock: controlling focus independently
Gain control over where the camera focuses when you depress the shutter button half-way. Say you want to focus on the subject that is off to one side in a scene. Move your digital camera so the focus area indicator is on the subject. Then press the shutter button down half way to lock focus. While holding the button in this position, recompose and then fully depress the button to take your shot.
This helps ensure that the main subject is in sharp focus rather than a random object in the scene selected by the camera.
AE-Lock: controlling exposure independently
Obtaining better exposed photos in difficult lighting situations involves moving the camera to the main area you want properly exposed. It should of equal distance as the subject since the shutter button controls focus too.
For example, if you want to capture detail in an area that would otherwise come out dark, lock exposure on the area, recompose and press the shutter-release button all the way down.
FE-Lock: controlling flash exposure
Flash Exposure-Lock locks exposure when taking photos with the flash. FE Lock can be very useful, as the following example illustrates.
If taking a photo of a person inside the house with a window behind them, turn on the flash and use FE-Lock to lock flash exposure on the face. The camera adjusts the flash output accordingly and the flash exposure will not be effected by the backlighting from the window.
Check your camera manual to see if your camera has these features. And, like all camera techniques, learn them by taking plenty of practice shots.