Lenses are measured in 'Focal Lengths'. You may have a 250mm lens or a 18mm lens, or you may have a zoom lens which can change its focal length and is may be listed like so: 50-250mm. (A lens that has a fixed focal length is called a 'prime lens')
In simple terms the focal length determines the magnification, a 250mm lens(telephoto) will magnify the subject much more than a 18mm(wide angle lens). But say you took a picture of a subject with each type of lens and each time you made the subject the same size with respect to the whole frame(this is achieved by changing the distance from the subject with the different focal lengths)
With the higher focal length you get a much narrower field of view and a much flatter image, with the wide lens you will see much more background(wider field of view)but with a lot more perspective and distortion)
So choosing which focal length to use should be about a lot more than simply deciding how close you want to get to the subject.
here is a gif that illustrates this(taken from reddit/photgraphy):
A more detailed explanation of focal lengths will come soon.