Sometimes LaTeX saves data it will reread later. These data are often the argument of some command; they are the so-called moving arguments. (“Moving” because data are moved around.) Candidates are all arguments that may go into table of contents, list of figures, etc.; namely, data that are written to an auxiliary file and read in later. Other places are those data that might appear in head- or footlines. Section headings and figure captions are the most prominent examples; there's a complete list in Lamport's book (see TeX-related books).
What's going on really, behind the scenes? The commands in moving
arguments are normally expanded to their internal structure during the
process of saving. Sometimes this expansion results in invalid TeX
code, which shows either during expansion or when the code is
processed again. Protecting a command, using
\protect\cmd tells LaTeX to save
\cmd, without expanding it at all.
So, what is a “fragile command”? - it's a command that expands into illegal TeX code during the save process.
What is a “robust command”? - it's a command that expands into legal TeX code during the save process.
Lamport's book says in its description of every LaTeX command whether it is “robust” or “fragile”; it also says that every command with an optional argument is fragile. The list isn't reliable, and neither is the assertion about optional arguments; the statements may have been true in early versions of LaTeX2e but are not any longer necessarily so:
\cite, have been made robust in later revisions of LaTeX.
\nocite, are fragile even though they have no optional arguments.
\newcommand*) now always creates a robust command (though macros without optional arguments may still be fragile if they do things that are themselves fragile).
\citecommand commonly suffers this treatment).
Further, simply “hiding” a fragile command in another command, has
no effect on fragility. So, if
\fred is fragile, and you write:
\jim is fragile too. There is, however, the
always creates a robust command (whether or not it has optional
arguments). The syntax of
\DeclareRobustCommand is substantially
identical to that of
\newcommand, and if you do the wrapping
trick above as:
\jim is robust.
Finally, we have the makerobust package, which defines
\MakeRobustCommand to convert a command to be robust. With the
package, the “wrapping” above can simply be replaced by:
\fred is robust. Using the package may be reasonable
if you have lots of fragile commands that you need to use in moving
In short, the situation is confusing. No-one believes this is
satisfactory; the LaTeX team have removed the need for
protection of some things, but the techniques available in
current LaTeX mean that this is an expensive exercise. It remains
a long-term aim of the team to remove all need for