Straight lines…
Lens distortion is when a straight line in reality is not represented as a straight line in your image. It is caused by the lens and the way the lens glass elements shape the light on its way to the sensor or film in your camera.
Modern lens constructions seldom have significant lens distortion, as the lens designers know that reviewers by tradition will test the lens for distortion and make a point of it if the lens suffers from distortion. In reality, most of us will never notice the distortion, and only if you shoot architecture, real estate or landscapes with edge-to-edge horizons will you notice the potential distortion in your lens.
Wide lenses, i.e. lenses with a short focal length, suffers more from distortion than longer lenses. The wide field of view that a wide lens has makes it more of a challenge for lens designers to make the edges of the image free from distortion. A good example where the designers deliberately have given up is a so called fish eye lens (say 8mm focal length), where the distortion to a large extend is the point.
In Lightroom and similar editing tools, the lens profile is available to the software and hence the distortion can easily be corrected in post, just by ticking off an option during post processing. In some cases the camera and the lens in combination is able to remove the distortion as part of the cameras internal processing of the image, and hence the image that you find on the memory card of your camera is distortion free from birth.
Thank you for reading this far! Comments and questions more than welcome!
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