Class field theory studies finite-dimensional abelian field extensions of number fields and of function fields, hence of global fields by relating them to the idele class group.
Class field theory clarifies the origin of various reciprocity laws in number theory. The basic (one dimensional) class field theory stems from the ideas of Kronecker and Weber, and results of Hilbert soon after them. Main results of the theory belong to the first half of the 20th century (Hilbert, Artin, Tate, Hasse…) and are quite different for the local field from the global field case. One of the basic objects the class group, is related to the Picard group in algebraic geometry. Generalizations for higher dimensional fields came later under now active higher class field theory, which is usually formulated in terms of algebraic K-theory and is closely related to deep questions of algebraic geometry (Paršin, Tate, Kato, Saito etc.).
Given an algebraic number field $k$ one defines a (congruence divisor) class field group $A/H$ in $k$; according to Weber
An algebraic extension $K/k$ is called a class field to $A/H$, if exactly those prime divisors in $k$ of first degree which belong to the principal class $H$ split completely in $K$
Some of the basic results of the class field theory are the Artin reciprocity theorem, existence theorem, uniqueness theorem, ordering theorem, Weber isomorphy theorem and the decomposition law of class field theory.
A. Fröhlich, J. W. S. Cassels (editors), Algebraic number theory, Acad. Press 1967, with many reprints; Fröhlich, Cassels, Birch, Atiyah, Wall, Gruenberg, Serre, Tate, Heilbronn, Rouqette, Kneser, Hasse, Swinerton-Dyer, Hoechsmann, systematic lecture notes from the instructional conference at Univ. of Sussex, Brighton, Sep. 1-17, 1965.
Albrecht Fröhlich, Martin J. Taylor, Algebraic number theory, Cambridge Studies in Advanced Mathematics 27, 1993
wikipedia class field theory
A. N. Paršin A. N. Local class field theory, Trudy Mat. Inst. Steklov 165 1984; Galois cohomology and Brauer group of local fields, Trudy Mat. Inst. Steklov 183, 1984.
The following survey of Connes-Marcolli work has a quick introduction to algebraic number theory including basic notions of CFT
The following article sketches the geometric intuition behind the reciprocity laws as the relation between two approaches to the maximal abelian quotient of the fundamental group, mimicking the ideas of Galois theory
The following 2 articles make parallel between some notions of QFT and of number theory and in particular about the analogy between the Weil reciprocity law for function fields and the Takahashi-Ward identities of field theory:
Some history is in