  We discuss in detail the $1+1$-dimensional superconformal field theory dual
to type II string theory on $AdS_3\times S^3\times T^4$, emphasizing the string
theoretic aspects of this duality. For one unit of NS-NS 5-brane flux
($Q_5=1$), this string theory has been suggested to be dual to a
grand-canonical ensemble of $T^{4N}/S_N$ free symmetric orbifold CFTs. We show
how the string genus expansion emerges to all orders for the free orbifold
grand-canonical correlation functions. We also discuss how the strong coupling
limit of the NS-NS string theory arises (even at large $N$) in the free
orbifold description, and argue why this limit does not have a weakly coupled
R-R description. The dual CFT includes (for all values of $Q_5$) an extra $T^4$
factor that is decoupled from perturbative string theory. We discuss the
exactly marginal deformations that relate the different values of $Q_5$,
including the precise $J{\bar J}$ deformations mixing this extra $T^4$ with the
symmetric orbifold.
