abelian transitive subgroups

Can anybody tell me what is known about the classification of abelian transitive groups of the symmetric groups?

For instance:

Let $G$ be a an abelian transitive subgroup of the symmetric group $S_n$. Why does $G$ have order $n$?

You mean, abelian subgroups of $S_n$ that act transitively on $\{1,\ldots,n\}$? – Arturo Magidin Apr 4 '12 at 19:26