Here's what ancestry.com has to say in their description of the Index:
About American Genealogical-Biographical Index (AGBI)
One of the most important genealogical collections, the American Genealogical-Biographical Index, or AGBI, is the equivalent of more than 200 printed volumes. This database contains millions of records of people whose names have appeared in printed genealogical records and family histories. With data from sources largely from the last century, each entry contains the person's complete name, the year of the biography's publication, the person's state of birth (if known), abbreviated biographical data, and the book and page number of the original reference. In addition to family histories, other genealogical collections are indexed. These include the Boston Transcript (a genealogical column widely circulated), the complete 1790 U.S. Federal Census, and published Revolutionary War records. The most recent update to this database reflects the inclusion of volumes 196-206. For researchers of American ancestors, this can be one of the most valuable databases available at Ancestry.com.
Most of the works referenced in the AGBI are housed at the Godfrey Memorial Library in Connecticut. A photocopy service is available. Please contact Godfrey Memorial Library at 134 Newfield St, Middletown, CT 06457 or via e-mail at firstname.lastname@example.org
to make use of this service.
To learn more about the AGBI, read the extended description below, but also read Kory Meyerink's article "Genealogy's Best-kept Secret: American Genealogical-Biographical Index."
Information about This Index:
For Biography—For use in biographical searches, this index presents few problems. It is an easy way to find information about the lives of many American men and women—information other sources do not always contain.
For Genealogy—Family historians will find this a tool unparalleled by any other. It should be one of the first tools used by genealogical researchers. This does not mean genealogists will always find the answer to every query; however, the researcher who fails to use the index may miss key information easily available. Nearly half of all references within the AGBI do not appear in any other place.
Information that Appears Nowhere Else—Ten percent of all published genealogies are not indexed anywhere else. This index includes Boston Transcript entries, with 2 million or more personal name references that appeared in the Boston Transcript during its forty-plus years of publication. Most of this material has never been published anywhere else.
Consolidation of Multiple Indexes—All of the twelve volumes of the First Census of the United States can be found here. This means the index offers a complete record, in one place, of all heads of families who lived in the United States in 1790. Also included are the forty-three volumes of records for Colonial soldiers who served in the Revolutionary War.
Names that Were Indexed—The index is of all persons according to set standards, rather than every name. The following persons have been included in the index: (1) person mentioned as wife, husband, father, mother, son, daughter, or other relative, of some person mentioned; (2) person mentioned as being born or married, or those mentioned dying; (3) person mentioned as having performed military or public service, or mentioned in connection with other facts of biographical importance; (4) person mentioned in a deed or legal document; (5) person mentioned as one of the founders of a settlement, a passenger on an immigrant ship (before 1850), a member of a church (before 1850), etc.
Name the Were Omitted—Omissions include: (1) persons (such as ship captains, ministers, army officers, etc.) mentioned only casually and not related to the family line being followed; (2) all casually mentioned names of well-known persons (e.g., George Washington or Benjamin Franklin); (3) witnesses, and similar incidental names, that appear in legal documents; (4) authors of works cited, or persons cited as authorities for statements.
Entry Construction—Each entry consists of the following: (1) Person's surname, spelled as it appears in the indexed text (Note that names are, in general, written and filed as one word, e.g., "Van Derbilt" and "Van Der Bilt" would be written as "Vanderbilt"; also, surnames with apostrophes have been indexed and alphabetized without the apostrophe, though it does appear in the actual name, e.g., "O'Connor" would be filed as "Oconnor."); (2) The person's first name (or initial) and middle names (or initials), if any (Note that if there is no given name, we have substituted a long dash in that area, and where an abbreviated name is given in the text, we have substituted the full name indicated if it is clear (e.g., for "Dan" we write "Daniel"); (3) The person's birth year, as it appears in the indexed text; (4) The person's state (or states) of residence (including the states of birth and death, if they are known); (5) Biographical data, abbreviated; (6) The page citation of the text being indexed; consisting of the abbreviated title and page number.
Abbreviations—State name abbreviations are uniformly two-letter. For well-known foreign countries, three-letter abbreviations are used (e.g., Can for Canada or Eng for England). For military service, the following abbreviations have been used (Note that if a man's rank is not stated, abbreviations "mil." or "nav." are used to indicate the branch of service): priv. - Private; lt. - Lieutenant; corp. - Corporal; serg. - Sergeant; comma. - Commander; capt. - Captain; maj. - Major; col. - Colonel; gen. - General; ens. - Ensign. If a child died young, or if a man or woman died unmarried or left no children, these facts are stated using these respective abbreviations: d.y. or d.inf., d.unm., or no ch.
Notes on the Compiling of an Index:
The efficient indexing of a genealogical work requires practical commonsense, as well as good editorial judgment. At times it also requires something approaching detective ability, for genealogies have been compiled by all sorts of people, most of whom have not had any previous experience in writing, and the material in some of them is devious and obscure to the last degree to everyone except the person who compiled it. If, in a given text, a genealogical descent is obscure or ambiguous, we decipher it and index the names cited according to the best conclusions we can arrive at without undue delay or abnormal cost. We are indexing, not unraveling obscurities.
Remember that the AGBI is an index to individuals appearing in family histories and other genealogical works, and researchers are encouraged to locate and examine the material from which the database was created. If not available from your local academic or research library, most of the works referenced in the AGBI are housed at the Godfrey Memorial Library in Connecticut. A photocopy service is available. Please contact Godfrey Memorial Library, 134 Newfield St, Middletown, CT 06457 or via e-mail at email@example.com