Flannel family part 3: Aperture

Aperture is the hole in your lens which light passes through.
It is more commonly used in the terms of f-stop number.
The f-stop number is the ratio of the focal length over the aperture diameter.

There common f-stop numbers are: 1.2, 1.4, 1.8, 2, 2.8, 4, 5.6, 8.
For the numbers 1.4, 2, 2.8, 4, 8, there is half as much light passing through the lens in each increasing number.
Setting a low fstop will give you more light, which lets you use a faster shutter speed.
It also narrows the depth of focus, so you can have only a small area in focus, and the background and foreground have a blur.
The quality of the blur is called bokeh.  Sometimes bokeh is confused with the blur itself.  Bokeh is the aesthetic quality of the blur.  Sometimes, due to the lens construction, there can be a blur that has a lot of jagged lines in the background.  This usually isnt considered good bokeh.  The shapes of light sources in your blurs are determined by the blades in your lens.  These blades control the aperture size, and "blink" when a camera takes a picture.  Cheaper lenses will use less blades.  The Canon 50mm f1.8 uses 5 blades, so when you have light sources in your bokeh, you will see pentagon shaped blurs.

One common mistake of the beginning photographer is that he or she will always shoot at the lowest fstop number to achieve that blur.  This fails when shooting either groups of people or a single subject at too close of a distance.  When shooting large groups, only the people standing in the same plane as the focused subject will be in focus.  When shooting a single subject, only a small sliver of their face will be in focus, such as the vertical plane running through their eyes.  Meanwhile, their nose or their ears will be out of focus.  Typically, one should close portraits or group portraits at f5.6, as this will widen the depth of focus to capture everyone/everything sharpley