Most people think of the standard lens as the old 50mm, but what do we mean by a standard lens. Well there are a number of definitions.
Most of the books state that a standard lens is one that produces a field of view similar the human eye.
A simple statement except that the human eye and brain is a decidedly different concept. We can concentrate on a small object in the landscape or sweep our eyes across a majestic vista. This gives us the impression of going from a telephoto to a wide angle in a single moment. The camera is a much more simple affair.
One is often better to consider perspective. It turns out that a lens of a focal equal to that of the diagonal of the 35mm piece of film (or full frame sensor) produces a very natural perspective and field of view.
So what focal length should a standard lens be; well it turns out its about 43mm. As far as I am aware only one manufacture made a 43mm lens and that was Pentax.
So a standard lens could be considered either a 35mm or 50mm lens. Depending on whether you want something slightly wider or longer.
For Medium Format users then they generally use a 75mm standard lens, and for Large format users, well thats a topic for another post.