Both film and digital cameras use aperture blades to control how much light hits the sensor or film. The blades open and close to make hole the light travel through larger or smaller. The best way to imagine a camera aperture is this: You've probably seen the opening of the James Bond movies where he turns to face the audience and fires his gun. You are looking at Bond in a camera aperture.The size of the aperture is measured in F-stops. The lower the F-stop number is, the larger or wider the aperture is. A wide F-stop is used for two reasons, either to create a nice Bokeh or background blur or to enable the use of a fast shutter speed to freeze the action. A narrow F-stop ensures that the whole image, from back to front, is all in focus. There are a multitude of variations in between the two, and clever control of the aperture size helps to produce the most stunning images. Not all cameras have an Aperture Priority mode, but if your camera has a mode dial with P A S M or P A Tv Av on it, it will have. The three images below are examples of different Aperture settings. Click to enlarge it like just like on Facebook.
Getting Started In Aperture Mode.
Turn Your camera dial to the A or Av position and turn the camera on. Check your manual to see how to set this value. On a compact, you should have a button to press to bring up a sliding scale which you set by using your cameras D-pad. On more expensive bridge cameras, there might be a second dial, called a 'Command Dial'  that you turn to the left or right to change the setting. It could be a dial like the mode dial, or like ones set into the body that you rotate, rather like old radios had inset into the top for a volume or tuning contol with a ridged edge, or a ring around the D-pad that turns left and right. Choose a low number (wide Aperture) for an image that's sharp at the front and blurred at the rear, or a high number (narrow Aperture) for an image that's sharp throughout the image. There's a basic chart in the next text box to aid you. 
Aperture is measured in F-stops. Each full F stop is equivelent to halving the size of the open aperture. F2 is half the width of F1. You can split each stop up into halves, thirds or quarters too. Most modern cameras will split an F-stop into halves or thirds. A basic F-stop list in halves is (put F: before the number) 0.7,. 0.8, 1.0, 1.2, 1.4, 1.7, 2.0, 2.8, 3.3, 4.0, 4.8, 5.6, 6.7, 8.0, 9.5, 11.0, 13.0, 16.0, 19.0, 22.0, 27.0 and 32.0.Apertures of under F5 are classed as wide, between 5 and F9.5 are mid range, whilst F and above are narrow apertures. Not all cameras go as high as F32, with most having a top stop of between F11 and F16, whilst some can only manage F5.6 at the top end. Generally, the maximum (widest) Fstop available reduces the more you zoom in. My Fuji HS30 manages a wide F2.8 at 24mm but only F5.6 at the telephoto end. Panasonic produce a superzoom bridge with a constant F2.8 across the whole of the range, albeit at the expense of a minimum (narrow) F stop.
As a rule, on bright sunny days, unless you want good bokeh, F16 should produce a good exposure, whilst cloudy days neccessitate an aperture of F8. Take a test shot and adjust the aperture wider or narrower as needed. The aperture priority mode will select an appropriate shutter speed. However, setting a narrow aperture may force the camera into setting a slow shutter speed to let enough light in, causing a blurred image. By way of contrast, setting a wide aperture (especially on a cloudy or dark day) may force the camera to set a fast shutter speed and give you a dark image. The above image which is sharp from back to front (taken on a partly cloudy day) was taken with an aperture of F9 and and a shutter speed of 1/320th of a second. An aperture of F5.6 or wider would have blurred the crowd in the background. A little exposure compensation of 2/3 of a stop was added to achieve the best exposure. Both Aperture and Shutter priority modes allow you to adjust the Exposure Compensation (a button with a square with +/- on it also known as EV). Depending on the conditions (sunshine, cloudy, night or low light), you might have to adjust this to get a correct exposure.  
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