Gedcom is a very loose set of tags which can be used to label data about a person.
For example, INDI is used to specify an individual numerically in the database, NAME the person's name. The context/meaning is generally obvious: SEX, OCCU, MARR, SPOUSE, etc.
Levels are indicated by numerals. For example, the line
1 BIRTcould be followed by the lines
2 DATE Jan 3, 1988 2 PLACE Worcester, MA, USAto indicate the subsidiary nature of that date and place. Order is not particulary critical. SPOUSE could come before or after FATHER.
[The source code for book-latex.ll (which we shall be using later) contains a list of useages it prefers.]