What is aperture, and how does it affect my photographs?
The aperture is the opening through which light goes to reach the camera's sensor.
The size of the aperture affects not only the amount of time required to take a photo, but also the depth of field within it.
With a wide aperture (so a low number, like f/1.8) gives a shallow depth of field - sometimes less than a millimetre with a macro lens. Because a lot of light is reaching the sensor (be it film or digital), this allows for fast shutter speeds
With a narrow aperture (so a high number like f/22), the depth of field is much greater, which is useful for things like landscape photography - it will limit the amount of light reaching your sensor, so you will get slower shutter speeds, which makes a tripod handy.
It's the size of the opening through which light goes to reach the camera's sensor.
Narrow aperature can cause difraction and limit the sharpness in certain cases. http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm Wide open aperature may also exacerbate various defects in the lense, such as chromatic aberation, sharpness, etc... I don't pretend I understand fully the physics why, but I think it has something to do with light hitting the edge of the lenses, which bends the light more and makes defects more visible.