  The concept of ``countable set'' is attributed to Georg Cantor, who set the
boundary between countable and uncountable sets in 1874. The concept of
``computable set'' arose in the study of computing models in the 1930s by the
founders of computer science, including \Godel, Church, and Turing. However,
the connection between countability and computability has not been thoroughly
studied in the past eight decades. A {\em counting bijection} of a set $S$ is a
bijection from the set of natural numbers to $S$. We say $S$ is {\em
enumerable} if either $S$ is finite or $S$ has a computable counting bijection.
We prove that $S$ is enumerable if and only if $S$ is computable. This
equivalence provides insights to the study of set theory and computability
theory and reveals that the concept of countability is related to space and
time complexity. We show that the set of total computable functions (or
deciders, algorithms) is unenumerable. We also provide a sufficient and
necessary condition for a set to be enumerable and use the concept of {\em
counting order} to explain why some sets are unenumerable. We show that some
popular statements about countable sets, such as ``every subset of a countable
set is countable'' or ``every formal language is countable,'' lack a valid
proof. These unproven statements are widely taught in college courses on
discrete mathematics, set theory, and computability theory.
