Quantum field theory (QFT) is a theoretical framework for constructing quantum mechanics|quantum mechanical models of field (physics)|fields and Many-body problem|many-body systems (in a Condensed matter physics|condensed matter context), both of which are systems classically represented by an infinite number of degrees of freedom (physics and chemistry)|degrees of freedom. They are also used in the description of critical phenomena and quantum phase transitions, such as in the BCS theory of superconductivity. Quantum field theories are especially useful for describing systems where the particle count/number may change over the course of a reaction.Most theories in modern particle physics are formulated as Special relativity|relativistic quantum field theories, such as quantum electrodynamics (QED), quantum chromodynamics (QCD), and the Standard Model. The quantum field-theoretic description of the electromagnetic field, quantum electrodynamics, approximately reproduces James Clerk Maxwell|Maxwell's theory of electrodynamics in the low-energy limit, with Euler-Heisenberg Lagrangian|small non-linear corrections to the Maxwell equations being required due to virtual particle|virtual electron-positron pairs.
Within a theory, there is one field for each type of particle in that theory, and interaction terms between the fields. For example, QED has one electron field and one photon field; QCD has one field for each type of quark, etc. The interaction terms are similar in spirit to those in Maxwell's equations, being interactions between fields. However unlike the classical fields of Maxwell's theory, QFT fields generally exist in Quantum superposition|superpositions of states.
In Perturbation theory (quantum mechanics)|perturbative quantum field theory, the forces between particles are mediated by other particles. The electromagnetic force between two electrons is caused by an exchange of photons. Intermediate vector bosons mediate the weak force and gluons mediate the strong force. Many of the quantum gravity|proposed theories to describe gravity as a QFT postulate the existence of a graviton particle that mediates the gravitational force. These force-carrying particles are virtual particles and, by definition, cannot be detected while carrying the force, because such detection will imply that the force is not being carried. In addition, the notion of force mediating particle comes from perturbation theory, and thus does not make sense in a context of non-perturbative bound states.
In QFT, photons are not thought of as little billiard balls but are rather viewed as field quanta – necessarily chunked ripples in a field, or excitations, that look like particles. Fermions, like the electron, can also be described as ripples/excitations in a field, where each kind of fermion has its own field. In summary, the classical visualisation of everything is particles and fields, in quantum field theory, resolves into everything is particles, which then resolves into everything is fields. In the end, particles are regarded as excited states of a field (field quanta). The gravitational field and the electromagnetic field are the only two fundamental fields in Nature that have infinite range and a corresponding classical low-energy limit, which greatly diminishes and hides their particle-like excitations. Albert Einstein, in 1905, attributed particle-like and discrete exchanges of momenta and energy, characteristic of field quanta, to the electromagnetic field. Originally, his principal motivation was to explain the thermodynamics of radiation. Although it is often claimed that the Photoelectric effect|photoelectric and Compton scattering|Compton effects require a quantum description of the electromagnetic field, this is now understood to be untrue, and proper proof of the quantum nature of radiation is now taken up into modern quantum optics as in the Photon antibunching|antibunching effect. The word photon was coined in 1926 by physical chemist Gilbert Newton Lewis (see also the articles photon antibunching and laser).
There is currently no complete quantum theory of the remaining fundamental interaction|fundamental force, gravity.
Presumably, the as yet unknown correct quantum field-theoretic treatment of the gravitational field will become and look exactly like Einstein's general theory of relativity in the low-energy limit, or, more generally, like the Einstein-Yang-Mills-Dirac System. Indeed, quantum field theory itself is possibly the low-energy-effective-field-theory limit of a more fundamental theory such as superstring theory. Compare in this context the article effective field theory.

Quantum field theory (QFT) is a theoretical framework for constructing quantum mechanics|quantum mechanical models of field (physics)|fields and Many-body problem|many-body systems (in a Condensed matter physics|condensed matter context), both of which are systems classically represented by an infinite number of degrees of freedom (physics and chemistry)|degrees of freedom. They are also used in the description of critical phenomena and quantum phase transitions, such as in the BCS theory of superconductivity. Quantum field theories are especially useful for describing systems where the particle count/number may change over the course of a reaction.Most theories in modern particle physics are formulated as Special relativity|relativistic quantum field theories, such as quantum electrodynamics (QED), quantum chromodynamics (QCD), and the Standard Model. The quantum field-theoretic description of the electromagnetic field, quantum electrodynamics, approximately reproduces James Clerk...