A "subset" means you can make it out of the pieces in the original set. No matter what set you begin with, you always have the option to choose no pieces at all--that creates the null subset.
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It is a universal set
That's not true. All sets have zero or more elements. You can have a set with zero elements - the "empty set".
Yes.
Equivalent sets are sets that have the same cardinality. For finite sets it means that they have the same number of distinct elements.For infinite sets, though, things get a bit complicated. Then it is possible for a set to be equivalent to a proper subset of itself: for example, the set of all integers is equivalent to the set of all even integers. What is required is a one-to-one mapping, f(x) = 2x, from the first set to the second.
The set of elements that are elements of the two (or more) given sets is called the intersection of the sets.