Quantum field theory is a theoretical framework that promotes quantum mechanics into a relativistic theory. Historically, the first developed quantum field theory was quantum electrodynamics. Quantum mechanics is all about measurements, whose expectations are calculated as averages of operators in the state vector of the system.
Quatum theory starts with a basic concept example of a scalar. Although this example, has been less obvious application in nature, it has considerable didactical virtues because it allows an explanation of the structural aspects of quantum field theory without encumbering the exposition with the extra difficulties posed by spin and gauge invariance.