Infinite Descending Chain

Question

From Wiki :-  

Given a set S with a partial order ≤, an infinite descending chain is an infinite, strictly decreasing sequence of elements x₁ > x₂ > ... > x_n > ...

As an example, in the set of integers, the chain −1, −2, −3, ... is an infinite descending chain, but there exists no infinite descending chain on the natural numbers, as every chain of natural numbers has a minimal element.

 

I can't understand this point "  but there exists no infinite descending chain on the natural numbers, as every chain of natural numbers has a minimal element. "

Please Explain How this Point is correct?

 

In mathematics, a natural number is either a positive integer (1, 2, 3, 4, ...) or a non-negative integer (0, 1, 2, 3, 4, ...). The former definition is generally used in number theory, while the latter is preferred in set theory and computer science. 

Now while solving question which definition to use ? 

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