GNU Emacs Lisp Reference Manual

29.18: Text Properties

Each character position in a buffer or a string can have a text property list, much like the property list of a symbol (see Property Lists). The properties belong to a particular character at a particular place, such as, the letter T at the beginning of this sentence or the first o in foo---if the same character occurs in two different places, the two occurrences generally have different properties.

Each property has a name and a value. Both of these can be any Lisp object, but the name is normally a symbol. The usual way to access the property list is to specify a name and ask what value corresponds to it.

If a character has a category property, we call it the category of the character. It should be a symbol. The properties of the symbol serve as defaults for the properties of the character.

Copying text between strings and buffers preserves the properties along with the characters; this includes such diverse functions as substring, insert, and buffer-substring.