Is String Theory a Field Theory? Is String Theory a Field or Quantum Mechanical Theory of the String rather than a Particle? 
I should know this having studied this for a term, but we jumped into the deep end, without really covering the basics of the theory.
 A: String theory isn't a quantum field theory. See What is the stress-energy distribution of a string in target space? and Statistics and macrolocality in string theory. See Do we need a quantum deformation of the diffeomorphism group in string theory? for a contrary opinion.
A: According to the definition, a field assigns a value (classically; or a distribution quantum mechanically) to every point in the space(-time). So field theory deals with point-like excitations in the space(-time). String theory, thus, is not quite a field theory, since it's excitations are defined on extended objects. To better understand the difference, I would look at http://en.wikipedia.org/wiki/String_field_theory.
Also, another important difference to notice is that people consider a few fundamental particles interacting with each other when they do qft; however, in string theory there are an infinite number of fundamental excitations in the theory, leading to an infinite tower of fundamental particles.
A: In string field theory a string field creates string excitations from the vacuum that interact. Interactions are treated using perturbation theory. The theory uses string vertex operators and string propagators. SFT is definitely a quantum field theory, but not a point particle QFT. It is used mostly in the study of unstable branes, topological string theories and non-commutative geometry. The principal versions of SFT are Light-Cone SFT, Covariant BRST SFT & Witten’s SFT. 
In principle string theory should be formulated as a quantum field theory of strings, but due to technical reasons related to the difficulties inherent to the above string field theory formulations, or simply by the incomplete knowledge of the underlying theory, that most calculations in the string theory literature are done in the context of first quantized formulation or in low energy effective classical actions. So it can be said that string theory & M-theory are, in principle, quantum field theories for extended objects, even if calculations are not generally done in this formalism
