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Finite set : F = {1,2,3,4,5}
This means that the set contains somehow countable elements.

Infinite set: I = {i | i ∈ R}      (the set of all real numbers)
Infinite set means there are infinite elements in the set.

Null set symbolized by { } or ø : N = {n | n is a month with 32 days}
A null set does not contain any elements, in other words, it is an empyty set.