org-todo-keywords

org-todo-keywords is a variable defined in `org.el'.
Its value is

((sequence "TODO" "DONE"))

Documentation:
List of TODO entry keyword sequences and their interpretation.
This is a list of sequences.

Each sequence starts with a symbol, either `sequence' or `type',
indicating if the keywords should be interpreted as a sequence of
action steps, or as different types of TODO items. The first
keywords are states requiring action - these states will select a headline
for inclusion into the global TODO list Org-mode produces. If one of
the "keywords" is the vertical bar, "|", the remaining keywords
signify that no further action is necessary. If "|" is not found,
the last keyword is treated as the only DONE state of the sequence.

The command C-c C-t cycles an entry through these states, and one
additional state where no keyword is present. For details about this
cycling, see the manual.

TODO keywords and interpretation can also be set on a per-file basis with
the special #+SEQ_TODO and #+TYP_TODO lines.

Each keyword can optionally specify a character for fast state selection
(in combination with the variable `org-use-fast-todo-selection')
and specifiers for state change logging, using the same syntax that
is used in the "#+TODO:" lines. For example, "WAIT(w)" says that
the WAIT state can be selected with the "w" key. "WAIT(w!)"
indicates to record a time stamp each time this state is selected.

Each keyword may also specify if a timestamp or a note should be
recorded when entering or leaving the state, by adding additional
characters in the parenthesis after the keyword. This looks like this:
"WAIT(w@/!)". "@" means to add a note (with time), "!" means to
record only the time of the state change. With X and Y being either
"@" or "!", "X/Y" means use X when entering the state, and use
Y when leaving the state if and only if the *target* state does not
define X. You may omit any of the fast-selection key or X or /Y,
so WAIT(w@), WAIT(w/@) and WAIT(@/@) are all valid.

For backward compatibility, this variable may also be just a list
of keywords. In this case the interpretation (sequence or type) will be
taken from the (otherwise obsolete) variable `org-todo-interpretation'.

You can customize this variable.