The syntax class of a character describes its syntactic role. Each syntax table specifies the syntax class of each character. There is no necessary relationship between the class of a character in one syntax table and its class in any other table.
Each syntax class is designated by a mnemonic character, which serves as the name of the class when you need to specify a class. Usually, this designator character is one that is often assigned that class; however, its meaning as a designator is unvarying and independent of what syntax that character currently has. Thus, ‘\’ as a designator character always means “escape character” syntax, regardless of whether the ‘\’ character actually has that syntax in the current syntax table. See Syntax Class Table, for a list of syntax classes and their designator characters.
A syntax descriptor is a Lisp string that describes the syntax
class and other syntactic properties of a character. When you want to
modify the syntax of a character, that is done by calling the function
modify-syntax-entry and passing a syntax descriptor as one of
its arguments (see Syntax Table Functions).
The first character in a syntax descriptor must be a syntax class designator character. The second character, if present, specifies a matching character (e.g., in Lisp, the matching character for ‘(’ is ‘)’); a space specifies that there is no matching character. Then come characters specifying additional syntax properties (see Syntax Flags).
If no matching character or flags are needed, only one character (specifying the syntax class) is sufficient.
For example, the syntax descriptor for the character ‘*’ in C
". 23" (i.e., punctuation, matching character slot
unused, second character of a comment-starter, first character of a
comment-ender), and the entry for ‘/’ is ‘. 14’ (i.e.,
punctuation, matching character slot unused, first character of a
comment-starter, second character of a comment-ender).
Emacs also defines raw syntax descriptors, which are used to describe syntax classes at a lower level. See Syntax Table Internals.