Here is how tag syntax is defined for the most popular languages:

• In C code, any C function or typedef is a tag, and so are definitions of struct, union and enum. #define macro definitions, #undef and enum constants are also tags, unless you specify --no-defines when making the tags table. Similarly, global variables are tags, unless you specify --no-globals, and so are struct members, unless you specify --no-members. Use of --no-globals, --no-defines and --no-members can make the tags table file much smaller. You can tag function declarations and external variables in addition to function definitions by giving the --declarations option to etags.
• In C++ code, in addition to all the tag constructs of C code, member functions are also recognized; member variables are also recognized, unless you use the --no-members option. operator definitions have tag names like operator+. If you specify the --class-qualify option, tags for variables and functions in classes are named /class/::/variable/ and /class/::/function/. By default, class methods and members are not class-qualified, which facilitates identifying their names in the sources more accurately.
• In Java code, tags include all the constructs recognized in C++, plus the interface, extends and implements constructs. Tags for variables and functions in classes are named /class/./variable/ and /class/./function/.
• In LaTeX documents, the arguments for \chapter, \section, \subsection, \subsubsection, \eqno, \label, \ref, \cite, \bibitem, \part, \appendix, \entry, \index, \def, \newcommand, \renewcommand, \newenvironment and \renewenvironment are tags. Other commands can make tags as well, if you specify them in the environment variable TEXTAGS before invoking etags. The value of this environment variable should be a colon-separated list of command names. For example, @example TEXTAGS="mycommand:myothercommand" export TEXTAGS @end example specifies (using Bourne shell syntax) that the commands \mycommand and \myothercommand also define tags.
• In Lisp code, any function defined with defun, any variable defined with defvar or defconst, and in general the first argument of any expression that starts with (def in column zero is a tag. As an exception, expressions of the form (defvar /foo/) are treated as declarations, and are only tagged if the --declarations option is given.
• In Scheme code, tags include anything defined with def or with a construct whose name starts with def. They also include variables set with set! at top level in the file.

Several other languages are also supported:

• In Ada code, functions, procedures, packages, tasks and types are tags. Use the --packages-only option to create tags for packages only. In Ada, the same name can be used for different kinds of entity (e.g., for a procedure and for a function). Also, for things like packages, procedures and functions, there is the spec (i.e., the interface) and the body (i.e., the implementation). To make it easier to pick the definition you want, Ada tag names have suffixes indicating the type of entity: @table @samp
• /b package body.
• /f function.
• /k task.
• /p procedure.
• /s package spec.
• /t type. @end table Thus, M-x find-tag =RET bidule/b RET= will go directly to the body of the package bidule, while M-x find-tag =RET bidule RET= will just search for any tag bidule.
• In assembler code, labels appearing at the start of a line, followed by a colon, are tags.
• In Bison or Yacc input files, each rule defines as a tag the nonterminal it constructs. The portions of the file that contain C code are parsed as C code.
• In Cobol code, tags are paragraph names; that is, any word starting in column 8 and followed by a period.
• In Erlang code, the tags are the functions, records and macros defined in the file.
• In Fortran code, functions, subroutines and block data are tags.
• In Go code, packages, functions, and types are tags.
• In HTML input files, the tags are the title and the h1, h2, h3 headers. Also, tags are name= in anchors and all occurrences of id=.
• In Lua input files, all functions are tags.
• In makefiles, targets are tags; additionally, variables are tags unless you specify --no-globals.
• In Objective C code, tags include Objective C definitions for classes, class categories, methods and protocols. Tags for variables and functions in classes are named /class/::/variable/ and /class/::/function/.
• In Pascal code, the tags are the functions and procedures defined in the file.
• In Perl code, the tags are the packages, subroutines and variables defined by the package, sub, use constant, my, and local keywords. Use --globals if you want to tag global variables. Tags for subroutines are named /package/::/sub/. The name for subroutines defined in the default package is main::/sub/.
• In PHP code, tags are functions, classes and defines. Vars are tags too, unless you use the --no-members option.
• In PostScript code, the tags are the functions.
• In Prolog code, tags are predicates and rules at the beginning of line.
• In Python code, def or class at the beginning of a line generate a tag.
• In Ruby code, def or class or module at the beginning of a line generate a tag. Constants also generate tags.
• In Rust code, tags anything defined with fn, enum, struct or macro_rules!.

You can also generate tags based on regexp matching (Etags Regexps) to handle other formats and languages.