General Instruction of the Roman Missal

The General Instruction of the Roman Missal (GIRM)—in the Latin original, Institutio Generalis Missalis Romani (IGMR)—is the detailed document governing the celebration of Mass of the Roman Rite in what since 1969 is its normal form. Originally published in 1969 as a separate document, it is printed at the start of editions of the Roman Missal since 1970.
In the circumstances indicated in the motu proprio "Summorum Pontificum" of 2007, the Catholic Church still permits celebrations of Mass in accordance with the 1962 edition of the Roman Missal. Such celebrations are governed not by the General Instruction but by the 1960 Code of Rubrics, particularly its section "Rubricae generales Missalis Romani" (General Rubrics of the Roman Missal), and by the "Ritus servandus in celebratione Missae" (Rite to be observed in celebration of Mass).
The 1960 Code of Rubrics replaced the "Rubricae Generales Missalis", which had been in the Tridentine Roman Missal since its first edition in 1570 and had been amplified and revised by Pope Clement VIII in 1604. This had been supplemented, since the 1920 edition, by the "Additiones et Variationes in Rubricis Missalis ad normam Bullae "Divino afflatu" et subsequentium S.R.C. decretorum" (Additions and Variations to the Rubrics of the Missal in accordance with the Bull "Divino afflatu" and subsequent decrees of the Sacred Congregation of Rites), which indicated the changes in the Roman Missal that followed from the reform of the Roman Breviary by Pope Pius X.
The General Instruction of the Roman Missal also replaced the document that in the original Tridentine Roman Missal (1570) was called "Ritus servandus in celebratione Missarum" (Rite to be observed in celebration of Masses) and that, after being revised by Pope Clement VIII, appeared in editions from 1604 on in altered and amplified form under the title "Ritus servandus in celebratione Missae" (Rite to be observed in celebration of Mass). In his 1962 edition, Pope John XXIII had made some changes in this document.
In his apostolic exhortation "Sacramentum caritatis", Pope Benedict XVI stressed the importance of proper knowledge of the "General Instruction" not only for priests but also for the laity: 
Structure.
The "General Instruction" is arranged in nine chapters, preceded by a preamble. The chapter headings are:
Versions.
The Latin original may be consulted at a number of sites. The most easily legible on a computer screen is perhaps that of the Salesians of Don Bosco (German Salesians).
An English translation, but with adaptations for the United States, can be consulted at the appropriate web page of the United States Conference of Catholic Bishops' Committee on Divine Worship. The same translation, but with adaptations instead for England and Wales, may be found at the web site of the England & Wales Liturgy Office.