Every autumn I explain to a new stream of graduate students what a PhD degree is.
It is difficult to describe it in words.
Therefore, I use pictures.
Imagine a circle that contains all the knowledge that humanity has accumulated:

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When you finish elementary school, you know quite a bit:

By the time of secondary education, you know a little more:

With a bachelor's degree you get a specialty:

The master's degree deepens this specialty:

Reading scientific articles, you find yourself on the very edge of human knowledge:

Once you have reached the limit, you choose a research area:

For several years you have been trying to move the border:

Until one day she moves:

This convex thing you did is called a PhD:

Of course, now the world looks a little different:

However, do not forget about the big picture:
From the translator: I know that formally the PhD is not equal to the candidate of science. Link to the original article via @ 2j2e .